Physical Evidence Paper

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9 Firearms Tool Marks, and Other Impressions

Image courtesy of National Atlas

LEARNING OBJECTIVES

After studying this chapter, you should be able to:

• Describe techniques for rifling a barrel. • Recognize the class and individual characteristics of bullets and cartridge cases. • Understand the use of the comparison microscope to compare bullets and cartridge cases. • Explain the concept of the NIBIN database. • Explain the procedure for determining how far from a target a weapon was fired. • Identify the laboratory tests for determining whether an individual has fired a weapon. • Explain the forensic significance of class and individual characteristics to the comparison of tool mark, footwear, and tire impressions. • List some common field reagents used to enhance bloody footprints.

THE BELTWAY SNIPERS

During a three-week period in October 2002, ten people were killed and three others wounded as two snipers terrorized the region in and around the Baltimore/Washington metropolitan area. The arrest of John Allen Muhammad, 41, and Lee Boyd

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Malvo, 17, ended the ordeal. The semiautomatic .223-caliber rifle seized from them was ultimately linked by firearm tests to eight of the ten killings. The car that Muhammad and Malvo were driving had been specially adapted with one hole in the trunk through which a rifle barrel could protrude, so that a sniper could shoot from inside the slightly ajar trunk.

The major break in the case came when a friend of Muhammad’s called police suggesting that Muhammad and his friend Malvo were the likely snipers. Muhammad’s automobile records revealed numerous traffic stops in the Beltway area during the time of the shootings. Another break in the case came when Malvo called a priest to boast of a killing that had occurred weeks before, in Montgomery, Alabama. Investigators traced the claim to a recent liquor store holdup that left one person dead. Fortunately, the perpetrator of this crime left a latent fingerprint at the murder scene. Authorities quickly tracked the print to Malvo, a Jamaican citizen, through his fingerprints on file with the Immigration and Naturalization Service. A description of Muhammad’s car was released to the media, leading to tips from alert citizens who noticed the car parked in a rest area with both occupants asleep.

The motive for the shooting spree was believed to be a plot to extort $10 million from local and state governments. Muhammad was sentenced to death, and Malvo is currently serving life imprisonment without parole.

Just as natural variations in skin ridge patterns and characteristics provide a key to human identification, minute random markings on surfaces can impart individuality to inanimate objects. Structural variations and irregularities caused by scratches, nicks, breaks, and wear permit the criminalist to relate a bullet to a gun, a scratch or abrasion mark to a single tool, or a tire track to a particular automobile. Individualization, so vigorously pursued in all other areas of criminalistics, is frequently attainable in firearms and tool mark examination.

Although a portion of this chapter will be devoted to the comparison of surface features for the purpose of bullet identification, a complete description of the services and capabilities of the modern forensic firearms laboratory cannot be restricted to just this one subject, important as it may be. The high frequency of shooting cases means that the science of firearms identification must extend beyond mere comparison of bullets to include knowledge of the operation of all types of weapons, restoration of obliterated serial numbers on weapons, detection and characterization of gunpowder residues on garments and around wounds, estimation of muzzle-to-target distances, and detection of powder residues on hands. Each of these functions will be covered in this chapter.

firearms identification A discipline primarily concerned with determining whether a bullet or cartridge was fired by a particular weapon.

Types of Firearms

Generally, firearms can be divided into two categories: handguns and long guns. Handguns, or pistols, are firearms that are designed to be held and fired with one hand. The three most common types of handguns are single-shot handguns, revolvers, and semiautomatics. All handguns can be classified as single-action or double-action firearms. Single-action firearms require the hammer to be manually cocked backward each time before the trigger is pulled in order to fire. Double-action firearms cock the hammer when the trigger is pulled and then reload the firing chamber after the round is fired.

Single-shot pistols can fire only one round, or shot, at a time. Each round must be manually loaded into the chamber before firing.

The revolver features several firing chambers located within a revolving cylinder. As the revolver is fired, the cylinder can rotate clockwise or counterclockwise. Each firing chamber holds one cartridge, which is lined up with the barrel mechanically when the round is fired. The cartridge cases have to be manually ejected to reload the firing chambers. Swing-out revolvers feature a cylinder that swings out to the side of the weapon to be loaded (see Figure 9-1). Break-top revolvers are hinged so that both the barrel and the cylinder flip downward for loading. Solid-frame revolvers have no mechanism to uncover all the firing chambers at once. Instead a small “gate” at the back of the gun allows one chamber to be loaded at a time; the cylinder is then rotated, and the next chamber is loaded with a cartridge.

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FIGURE 9-1 A swing-out revolver features a cylinder that swings out to the side of the weapon to be loaded.

Semiautomatic pistols feature a removable magazine that is most often contained within the grip of the firearm. Once the magazine is loaded, the hammer is cocked by pulling the slide on the top of the gun rearward and then releasing it to load the first round. The firing of the cartridge generates gases that are used to eject the cartridge case, cock the hammer, and load the next round. A semiautomatic pistol (see Figure 9-2) fires one shot per trigger pull. An automatic firearm, such as a machine gun, fires as long as the trigger is pressed or until the ammunition is depleted.

Long guns are either rifles or shotguns. Rifles and shotguns are designed to be fired while resting on the shoulder. The two principal differences between rifled firearms and shotguns are found in the ammunition and the barrel. Shotgun ammunition, called a shell, contains numerous ball-shaped projectiles, called shot. The barrel of a shotgun is smooth without the grooves and lands found in rifles. A shotgun barrel can also be narrowed toward the muzzle in order to concentrate shot when fired. This narrowing of the barrel is called the choke of the shotgun. A shotgun may be single or double barreled. The two barrels of a double-barreled shotgun may be arranged horizontally (side by side) or vertically (one over another). The barrels may also have different choke diameters.

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FIGURE 9-2 A semiautomatic pistol.

The various types of rifles and shotguns have different reloading mechanisms. The single-shot gun can chamber and fire only one round at a time. Just as with single-shot pistols, the round has to be loaded manually each time. Repeating long guns use a mechanical instrument of some sort to eject spent cartridges, load a new round, and cock the hammer after a round is fired. These include lever-action, pump or slide-action, bolt-action (see Figure 9-3), and semiautomatic (see Figure 9-4) long guns, the names of which refer to the loading mechanism used on each. Semiautomatic rifles use the force of the gas produced during firing to eject the spent cartridge, load a new round, and cock the hammer. Semiautomatic firearms use a disconnector mechanism to fire one shot per trigger pull, whereas fully automatic firearms do not have such a mechanism and fire multiple consecutive shots with a single pull of the trigger.

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FIGURE 9-3 A bolt-action long gun uses the movement of a bolt mechanism to expel the spent cartridge case, load the next round, and cock the hammer.

Getty Images, Inc./Hulton Archive Photos

Bullet and Cartridge Comparisons

The inner surface of the rifled barrel of a gun leaves its markings on a bullet passing through it. These markings are particular to each gun. Hence, if one bullet found at the scene of a crime and another test-fired from a suspect’s gun exhibit the same markings, the suspect is linked to the crime. Because these inner surface markings, or striations, are so important for bullet comparison, it is important to know why and how they originate.

THE GUN BARREL

The gun barrel is produced from a solid bar of steel that has been hollowed out by drilling. The microscopic drill marks left on the barrel’s inner surface are randomly irregular and in themselves impart a uniqueness to each barrel. However, the manufacture of a barrel requires the additional step of impressing its inner surface with spiral grooves, a step known as

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rifling. The surfaces of the original bore remaining between the grooves are called lands (see Figure 9-5).

grooves The cut or low-lying portions between the lands in a rifled bore.

rifling The spiral grooves formed in the bore of a firearm barrel that impart spin to the projectile when it is fired.

bore The interior of a firearm barrel.

lands The raised portion between the grooves in a rifled bore.

As a fired bullet travels through a barrel, it engages the rifling grooves. These grooves then guide the bullet through the barrel, giving it a rapid spin. This is done because a spinning bullet does not tumble end over end on leaving the barrel, but instead remains on a true and accurate course.

FIGURE 9-4 A semiautomatic long gun uses the energy from the firing reaction to expel the spent cartridge case, load the next round, and cock the hammer.

Tim Ridley © Dorling Kindersley, Courtesy of the Ministry of Defence Pattern Room, Nottingham

The diameter of the gun barrel, shown in Figure 9-6, is measured between opposite lands and is known as the caliber of the weapon. Caliber is normally recorded in hundredths of an inch or in millimeters—for example, .22 caliber and 9 millimeter. Actually, the term caliber, as it is commonly applied, is not an exact measurement of the barrel’s diameter; for example, a .38-caliber weapon might actually have a bore diameter that ranges from 0.345 to 0.365 inch.

caliber The diameter of the bore of a rifled firearm, usually expressed in hundredths of an inch or millimeters—for example, .22 caliber and 9 millimeter.

RIFLING METHODS

Before 1940, barrels were rifled by having one or two grooves at a time cut into the surface with steel hook cutters. The cutting tool was rotated as it passed down the barrel, so that the final results were grooves spiraling to either the right or left. However, as the need for increased speed and efficiency in methods of weapons manufacture became apparent, newer techniques were developed that were far more suitable for mass production.

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The broach cutter, shown in Figure 9-7, consists of a series of concentric steel rings, with each ring slightly larger than the preceding one. As the broach passes through the barrel, it simultaneously cuts all grooves into the barrel at the required depth. The broach rotates as it passes through the barrel, giving the grooves their desired direction and rate of twist.

FIGURE 9-5 Interior view of a gun barrel, showing the presence of lands and grooves.

FIGURE 9-6 A cross-section of a barrel with six grooves. The diameter of the bore is the caliber.

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In contrast to the broach, the button process involves no cuttings. A steel plug or “button” impressed with the desired number of grooves is forced under extremely high pressure through the barrel. A single pass of the button down the barrel compresses the metal to create lands and grooves on the barrel walls that are negative forms of those on the button. The button rotates to produce the desired direction and rate of twist (see Figure 9-8).

FIGURE 9-7 A segment of a broach cutter.

Courtesy Susan Walsh, AP Wide W orld Photos

FIGURE 9-8 (top) Cross-section of a .22-caliber rifled barrel. (bottom) A button used to produce the lands and grooves in the barrel.

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Like the button process, the mandrel-rifling hammer-forging process involves no cutting of metal. A mandrel is a rod of hardened steel machined so its form is the reverse impression of the rifling it is intended to produce. The mandrel is inserted into a slightly oversized bore, and the barrel is compressed with hammering or heavy rollers into the mandrel’s form.

Every firearms manufacturer chooses a rifling process that is best suited to meet the production standards and requirements of its product. Once the choice is made, however, the class characteristics of the weapon’s barrel remain consistent; each has the same number of lands and grooves, with the same approximate width and direction of twist. For example, .32-caliber Smith & Wesson revolvers have five lands and grooves twisting to the right. On the other hand, Colt .32-caliber revolvers exhibit six lands and grooves twisting to the left. Although these class characteristics permit the examiner to distinguish one type or brand name of weapon from another, they do not impart individuality to any one barrel; no class characteristic can do this.

FIGURE 9-9 A bullet is impressed with the rifling markings of the barrel when it emerges from the weapon.

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If one could cut a barrel open lengthwise, careful examination of the interior would reveal the existence of fine lines, or striations, running the length of the barrel’s lands and grooves. These striations are impressed into the metal as the negatives of minute imperfections found on the rifling cutter’s surface, or they are produced by minute chips of steel pushed against the barrel’s inner surface by a moving broach cutter. The random distribution and irregularities of these markings are impossible to duplicate exactly in any two barrels. No two rifled barrels, even those manufactured in succession, have identical striation markings. These striations form the individual characteristics of the barrel.

COMPARING BULLET MARKINGS

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As the bullet passes through the barrel, its surface is impressed with the rifled markings of the barrel. The bullet emerges from the barrel carrying the impressions of the bore’s interior surface (see Figure 9-9). Because there is no practical way to directly compare the markings on the fired bullet and those within a barrel, the examiner must obtain test bullets fired through the suspect barrel for comparison. To prevent damage to the test bullet’s markings and to facilitate the bullet’s recovery, test firings are normally made into a recovery box filled with cotton or into a water tank (see Figure 9-10).

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FIGURE 9-10 In ballistics testing, a suspect firearm is fired into a water tank. The bullet is slowed and stopped by the water, fished out undamaged, and compared to bullets from the crime scene.

Courtesy Mikael Karlsson/Arresting Images

The number of lands and grooves and their direction of twist are obvious points of comparison during the initial stages of the examination. Any differences in these class characteristics immediately eliminate the possibility that both bullets traveled through the same barrel. A bullet with five lands and grooves could not possibly have been fired from a weapon of like caliber with six lands and grooves, nor could one having a right twist have come through a barrel impressed with a left twist. If both bullets carry the same class characteristics, the analyst must then begin to match the striated markings on both bullets. This can be done only with the assistance of a comparison microscope.

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Modern firearms identification began with the development and use of the comparison microscope. This instrument is the firearms examiner’s most important tool. The test and evidence bullets are mounted on cylindrical adjustable holders beneath the objective lenses of the microscope, each pointing in the same direction (see Figure 9-11). Both bullets are observed simultaneously within the same field of view, and the examiner rotates one bullet until a well-defined land or groove comes into view.

FIGURE 9-11 A bullet holder beneath the objective lens of a comparison microscope.

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Courtesy Leica Microsystems, Buffalo, NY, www.leica-microsystems.com

Once the striation markings are located, the other bullet is rotated until a matching region is found. Not only must the lands and grooves of the test and evidence bullet have identical widths, but the longitudinal striations on each must coincide. When a matching area is located, the two bullets are simultaneously rotated to obtain additional matching areas around the periphery of the bullets. Figure 9-12 shows a typical photomicrograph of a bullet match as viewed under a comparison microscope.

FIGURE 9-12 A photomicrograph of two bullets through a comparison microscope. The test bullet is on the right; the questioned bullet is on the left.

Courtesy Philadelphia Police Department Laboratory

CONSIDERATIONS IN BULLET COMPARISON

Unfortunately, the firearms examiner rarely encounters a perfect match all around the bullet’s periphery. The presence of grit and rust can alter the markings on bullets fired through the same barrel. More commonly, recovered evidence bullets become

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so mutilated and distorted on impact that they yield only a small area with intact markings.

Furthermore, striation markings on a barrel are not permanent structures; they are subject to continuing alteration through wear as succeeding bullets traverse the length of the barrel. Fortunately, these changes are usually not dramatic and do not prevent the matching of two bullets fired by the same weapon. As with fingerprint comparison, there are no hard-and-fast rules governing the minimum number of points required for a bullet comparison. The final opinion must be based on the judgment, experience, and knowledge of the expert.

Frequently, the firearms examiner receives a spent bullet without an accompanying suspect weapon and is asked to determine the caliber and possible make of the weapon. If a bullet appears not to have lost any metal, its weight may be one factor in determining its caliber. In some instances, the number of lands and grooves, the direction of twist, and the widths of lands and grooves are useful class characteristics for eliminating certain makes of weapons from consideration. For example, a bullet that has five lands and grooves and twists to the right could not have come from a weapon manufactured by Colt because Colts are not manufactured with these class characteristics.

Sometimes a bullet has rifling marks that set the weapon it was fired from apart from most other manufactured weapons, as is the case with Marlin rifles. These weapons are rifled by a technique known as microgrooving and may have eight to twenty-four grooves impressed into their barrels; few other weapons are manufactured in this fashion. For this reason, the FBI maintains a record known as the General Rifling Characteristics File. This file contains listings of class characteristics, such as land- and groove-width measurements, for known weapons. It is periodically updated and distributed to the law enforcement community to help identify rifled weapons from retrieved bullets.

Unlike rifled firearms, a shotgun has a smooth barrel, so projectiles passing through a shotgun barrel are not impressed with any characteristic markings that can be related back to the weapon later. Shotguns generally fire small lead balls or pellets contained within a shotgun shell (see Figure 9-13). A paper or plastic wad pushes the pellets through the barrel when the shell’s powder charge is ignited. By weighing and measuring the diameter of the shot recovered at a crime scene, the examiner can usually determine the size of shot used in the shell. The size and shape of the recovered wad may also reveal the gauge of the shotgun used and, in some instances, may indicate the manufacturer of the fired shell.

FIGURE 9-13 A cross-section of a loaded shotgun shell.

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CLOSER ANALYSIS THE COMPARISON MICROSCOPE

Forensic microscopy often requires a side-by-side comparison of specimens. This kind of examination can best be performed with a comparison microscope, such as the one pictured in the figure.

Basically, the comparison microscope is two compound microscopes combined into one unit. The unique feature of its design is that it uses a bridge incorporating a series of mirrors and lenses to join two independent objective lenses into a single binocular unit. A viewer looking through the eyepiece lenses of the comparison microscope observes a circular field equally divided into two parts by a fine line. The specimen mounted under the left-hand objective appears in the left half of the field, and the specimen under the right-hand objective appears in the right half of the field. It is important to closely match the optical characteristics of the objective lenses to ensure that both specimens are seen at equal magnification and with minimal but identical lens distortions. Comparison microscopes designed to compare opaque objects, such as bullets and cartridges, are equipped with vertical or reflected illumination. Comparison microscopes used to compare hairs or fibers use transmitted illumination.

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The comparison microscope—two independent objective lenses joined together by an optical bridge.

Courtesy Leica Microsystems, Buffalo, NY, www.leica-microsystems.com

Figure 9-12 shows the striation markings on two bullets that have been placed under the objective lenses of a comparison microscope. Modern firearms examination began with the introduction of the comparison microscope, which gives the firearms examiner a side-by-side, magnified view of two bullets. Bullets that are fired through the same rifle barrel display comparable rifling markings on their surfaces. Matching the majority of striations present on each bullet justifies a conclusion that both bullets traveled through the same barrel.

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The diameter of the shotgun barrel is expressed in terms of its gauge.1 The higher the gauge number, the smaller the barrel’s diameter. For example, a 12-gauge shotgun has a bore diameter of 0.730 inch, and a 16-gauge shotgun has an interior diameter of 0.670 inch. The exception to this rule is the .410-gauge shotgun, whose gauge number refers directly to the barrel’s bore measurement of 0.41 inch in diameter.

gauge The size designation of a shotgun; originally the number of lead balls with the same diameter as the barrel that would make a pound. The only exception is the .410 shotgun, in which bore size is 0.41 inch.

CARTRIDGE CASES

The act of pulling a trigger releases the weapon’s firing pin, causing it to strike the primer, which in turn ignites the powder. The expanding gases generated by the burning gunpowder propel the bullet forward through the barrel, simultaneously pushing the spent cartridge case or shell back with equal force against the breech face mark. As the bullet is marked by its passage through the barrel, the shell is also impressed with markings by its contact with the metal surfaces of the weapon’s firing and loading mechanisms. As with bullets, these markings can be reproduced in test-fired cartridges to provide distinctive points of comparison for individualizing a spent shell to a rifled weapon or shotgun.

breech face mark The rear part of a firearm barrel.

CASE FILES SACCO AND VANZETTI

In 1920, two security guards were viciously gunned down by unidentified assailants. The security guards were transporting shoe factory payrolls, nearly $16,000 in cash, at the time of the robbery-murder. Eyewitnesses described the assailants as “Italian-looking,” one with a full handlebar moustache. The robbers had used two firearms that left behind three different brands of shells.

Two suspects were identified and arrested: Nicola Sacco and his friend, the amply mustachioed Bartolomeo Vanzetti. After denying owning any firearms, each was found to be in possession of a loaded pistol. In fact, Sacco’s pistol was .32 caliber, the same caliber as the crime-scene bullets. In Sacco’s pockets were found twenty-three bullets matching the brands of the empty shells found at the murder scene.

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CORBIS-NY

This case coincided with the “Red Scare,” a politically turbulent time in post–World War I America. Citizens feared socialist zeal-ots, and the media played up these emotions. Political maneuvering and the use of the media muddied the waters surrounding the case, and the fact that both suspects belonged to anarchist political groups that advocated revolutionary violence against the government only incited public animosity toward them. Sympathetic socialist organizations attempted to turn Sacco and Vanzetti into martyrs, calling their prosecution a “witch hunt.”

The outcome of the trial ultimately depended on whether the prosecution could prove that Sacco’s pistol fired the bullets that killed the two security guards. At trial, the ballistics experts testified that the bullets used were no longer in production, and they could not find similar ammunition to use in test firings—aside from the unused cartridges found in Sacco’s pockets. A forensics expert for the prosecution concluded that a visual examination showed that the bullets matched, leading the jury to return a verdict of guilty. Sacco and Vanzetti were sentenced to death.

Because of continued public protests, a committee was appointed in 1927 to review the case. Around this time, Calvin Goddard, at the Bureau of Forensic Ballistics in New York, perfected the comparison microscope for use in forensic firearms investigations. The committee asked Goddard to examine the bullets in question. A test-fired bullet from Sacco’s weapon was matched conclusively by Goddard to one of the crime-scene bullets. The fates of Sacco and Vanzetti were sealed, and they were put to death in 1927.

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The cup of the firing pin is impressed into the relatively soft metal of the primer on the cartridge case, revealing the minute distortions of the firing pin. These imperfections may be sufficiently random to individualize the pin impression to a single weapon. Similarly, the cartridge case, in its rearward thrust, is impressed with the surface markings of the breech face mark. The breech face mark, like any machined surface, is populated with random striation markings that become a highly distinctive signature for individualizing its surface.

Other distinctive markings that may appear on the brass portions of shells as a result of metal-to-metal contact are caused by the extractor and ejector mechanism and the magazine, or clip, as well as by imperfections on the firing chamber walls. The photomicrographs in Figure 9-14 reveal a comparison of the firing pin and breech face mark impressions on evidence and test-fired shells.

extractor The mechanism in a firearm that withdraws a cartridge or fired case from the chamber.

ejector The mechanism in a firearm that throws the cartridge or fired case from the firearm.

These impressions provide points for individualizing the shell to a weapon that are just as valuable as cartridge cases discharged from a rifled firearm. Furthermore, in the absence of a suspect weapon, the size and shape of a firing pin impression and/or the position of ejector marks in relationship to extractor and other markings may provide some clue to the type or make of the weapon that fired the questioned shell, or at least eliminate a large number of possibilities.

FIGURE 9-14 A comparison microscope photomicrograph showing a match between (a) firing pin impressions and (b) the breech face mark on two shells.

Courtesy Ronald Welsh, Bureau of Forensic Services, Central Valley Laboratory, Ripon, CA

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Quick Review

Automated Firearms Search Systems

The use of firearms, especially semiautomatic weapons, during the commission of a crime has significantly increased throughout the United States. Because of the expense of such firearms, the likelihood that a specific weapon will be used in multiple crimes has risen. The advent of computerized imaging technology has made it possible to store bullet and cartridge surface characteristics in a manner analogous to automated fingerprint files. Using this concept, crime laboratories can be networked, allowing them to share information on bullets and cartridges retrieved from several jurisdictions.

EARLY SYSTEMS

The effort to build a national computerized database for firearms evidence in the United States had a rather confusing and inefficient start in the early 1990s. Two major federal law enforcement agencies, the FBI and the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), offered the law enforcement community competing and incompatible computerized systems.

The automated search system developed for the FBI was known as DRUG-FIRE. This system emphasized the examination of unique markings on the cartridge casings expended by the weapon. The specimen was analyzed through a microscope attached to a video camera. The magnification allowed for a close-up view to identify individual characteristics. The image was captured by the video camera, digitized, and stored in a database. Although DRUGFIRE emphasized cartridge-case imagery, the images of highly characteristic bullet striations could also be stored in a like manner for conducting comparisons.

The Integrated Ballistic Identification System (IBIS), developed for the Bureau of Alcohol, Tobacco, Firearms, and Explosives, processed digital microscopic images of identifying features found on both expended bullets and cartridge casings. IBIS incorporated two software programs: Bulletproof, a bullet-analyzing module, and Brasscatcher, a cartridge- case-analyzing module. A schematic diagram of Bulletproof’s operation is depicted in Figure 9-15.

FIGURE 9-15 Bulletproof configuration. The sample is mounted on the

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specimen manipulator and illuminated by the light source from a microscope. The image is captured by a video camera and digitized. This digital image is then stored in a database, available for retrieval and comparison. The search for a match includes analyzing the width of land and groove impressions along with both rifling and individual characteristics. The Brasscatcher software uses the same system configuration but emphasizes the analysis of expended cartridge casings rather than the expended bullets.

Courtesy Forensic Technology (WAI) Inc., Côte St-Luc, Quebec, Canada

NIBIN

In 1999, members of the FBI and ATF joined forces to introduce the National Integrated Ballistics Information Network (NIBIN) program to the discipline of firearms examination. The new, unified system incorporates both DRUGFIRE and IBIS technologies. ATF has overall responsibility for the system sites, and the FBI is responsible for the communications network.

Agencies using the new NIBIN technology produce database files from bullets and cartridge casings retrieved from crime

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scenes or test fires from retrieved firearms. More than two hundred law enforcement agencies worldwide have adapted this technology. The success of the system has been proved: With more than 1.6 million images compiled nationwide, law enforcement agencies have connected more than 34,000 bullets and casings to more than one crime (see Figure 9-16).

FIGURE 9-16 Bullets A, B, C, and D were acquired by the IBIS database at different times from different crime scenes. D is a fragmented bullet that had only three land impressions available for acquisition. On entering bullet D, IBIS found a potential matching candidate in the database: B. On the far right, bullet D is compared to bullet B using the IBIS imaging software. Finally, a forensic firearms examiner using the actual evidence under a conventional comparison microscope will confirm the match between B and D.

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Courtesy Forensic Technology (WAI) Inc., Côte St-Luc, Quebec, Canada

For example, in a recent case, a robbery-turned-double-homicide left two store clerks dead. Two bullets and two .40-caliber Smith & Wesson cartridge casings were recovered. Later that day, a Houston security guard was shot and killed during a botched armed robbery. A bullet and .40-caliber Smith & Wesson cartridge casing were recovered and entered into NIBIN. Once these were processed, a correlation was found between the murder of the security officer and a separate aggravated robbery that had occurred two weeks earlier. All three crimes were linked with a firearm believed to be a .40-caliber Smith & Wesson pistol.

Further investigation into the use of a victim’s credit card helped police locate two suspects. In the possession of one suspect

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was a .40-caliber Smith & Wesson pistol. The gun was test-fired and imaged into NIBIN. The casing from the test-fired weapon matched the evidence obtained in the robbery and the aggravated robbery-homicides. A firearms examiner verified the associations by traditional comparisons. Before this computerized technology existed, it would have taken years—or it may have been impossible—to link all of these shootings to a single firearm.

In another example, the ATF laboratory in Rockville, Maryland, received 1,466 cartridge casings from the Ovcara mass burial site in Bosnia. After processing and imaging profiles for all casings, the examiners determined that eighteen different firearms had been used at the site. With the help of NIBIN technology and competent examiners, jurists attempted to convict an individual for war crimes.

NIBIN serves only as a screening tool for firearms evidence. A computerized system does not replace the skills of the firearms examiner. NIBIN can screen hundreds of unsolved firearms cases and may narrow the possibilities to several firearms. However, the final comparison will be made by a forensic examiner through traditional microscopic methods.

BALLISTIC FINGERPRINTING

Participating crime laboratories in the United States are building databases of bullet and cartridge cases found at crime scenes and those fired in tests of guns seized from criminals. As these databases prove their usefulness in solving crimes, law enforcement officials and the political community are scrutinizing the feasibility of scaling this concept up to create a system of ballistic fingerprinting. This system would entail the capture and storage of appropriate markings on bullets and cartridges test-fired from handguns and rifles before they are sold to the public. Questions regarding who will be responsible for collecting the images and details of how will they be stored are but two of many issues to be decided. The concept of ballistic fingerprinting is an intriguing one for the law enforcement community and promises to be explored and debated intensely in the future.

Quick Review

• The advent of computerized imaging technology has made possible the storage of bullet and cartridge surface characteristics in a manner analogous to automated fingerprint files. • Two automated firearms search systems are DRUGFIRE, developed by the FBI, and IBIS, developed by the ATF. • NIBIN is the National Integrated Ballistics Information Network, a unified firearms search system that incorporates both DRUGFIRE and IBIS technologies.

Gunpowder Residues

Modern ammunition is propelled toward a target by the expanding gases created by the ignition of smokeless powder or nitrocellulose in a cartridge. Under ideal circumstances, all of the powder is consumed in the process and converted into the rapidly expanding gases. However, in practice the powder is never totally burned. When a firearm is discharged, unburned and partially burned particles of gunpowder in addition to smoke are propelled out of the barrel, along with the bullet, toward the target. If the muzzle of the weapon is sufficiently close, these products are deposited onto the target. The distribution of gunpowder particles and other discharge residues around the bullet hole permits a distance determination, an assessment of the distance from which a handgun or rifle was fired.

distance determination The process of determining the distance between the firearm and a target, usually based on the distribution of powder patterns or the spread of a shot pattern.

DISTANCE DETERMINATION

In incidents involving gunshot wounds, it is often necessary to determine the distance from which the weapon was fired. For example, in incidents involving a shooting death, the suspect often pleads self-defense as the motive for the attack. Such

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claims are fertile grounds for distance determinations because finding the proximity of the people involved is necessary to establish the facts of the incident. Similarly, careful examination of the gunshot wounds of suicide victims usually reveals characteristics associated with a very close-range shot. The absence of such characteristics strongly indicates that the wound was not self-inflicted and signals the possibility of foul play.

The accuracy of a distance determination varies according to the circumstances of the case. When the investigator is unable to recover a suspect weapon, the best the examiner can do is to state whether a shot could have been fired a given distance from the target. More exact opinions are possible only when the examiner has the suspect weapon in hand and knows the type of ammunition used in the shooting.

HANDGUNS AND RIFLES

The precise distance from which a handgun or rifle has been fired must be determined by carefully comparing the powder residue pattern on the victim’s clothing or skin to test patterns made by firing the suspect weapon at varying distances from a target. A white cloth or a fabric comparable to the victim’s clothing may be used as a test target (see Figure 9-17). Because the spread and density of the residue pattern vary widely among weapons and ammunition, such a comparison is significant only when it is made with the suspect weapon and suspect ammunition, or with ammunition of the same type and make. By comparing the test and evidence patterns, the examiner may find enough similarity in shape and density by which to judge the distance from which the shot was fired.

Without the weapon, the examiner is restricted to looking for recognizable characteristics around the bullet hole. Such findings are at best approximations made as a result of general observations and based on the examiner’s experience. However, some noticeable characteristics should be sought. For instance, when the weapon is held in contact with or less than 1 inch from the target, a heavy concentration of residue from smokelike vaporous lead usually surrounds the bullet’s entrance hole. Often, loose fibers surrounding a contact hole show scorch marks from the flame discharge of the weapon, and some synthetic fibers may show signs of being melted as a result of the heat from the discharge. Furthermore, the blowback of muzzle gases may produce a stellate (i.e., star-shaped) tear pattern around the hole. Such a hole is invariably surrounded by a rim of a smokelike deposit of vaporous lead (see Figure 9-18).

FIGURE 9-17 Test powder patterns made with a Glock 9mm luger fired at the following distances: (a) contact, (b) 6 inches, (c) 12 inches, and (d) 18 inches.

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A halo of vaporous lead deposited around a bullet hole normally indicates the bullet was discharged 18 inches or less from the target. The presence of scattered specks of unburned and partially burned powder grains without any accompanying soot can often be observed at distances up to approximately 25 inches. Occasionally, however, scattered gunpowder particles are noted at a firing distance as far out as 36 inches. A weapon that has been fired more than 3 feet from a target usually does not deposit any powder residues on the target’s surface. (However, with ball powder ammunition, this distance may be extended to 8 feet.)

FIGURE 9-18 A contact shot.

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When a weapon has been fired from 3 feet or more away, the only visual indication that the hole was made by a bullet is a dark ring, known as bullet wipe, around the perimeter of the entrance hole. Bullet wipe consists of a mixture of carbon, dirt, lubricant, primer residue, and lead wiped off the bullet’s surface as it passes through the target. Again, in the absence of a suspect weapon, these observations are only general guidelines for estimating target distances. Numerous factors—barrel length, caliber, type of ammunition, and type and condition of the weapon fired—influence the amount of gunpowder residue deposited on a target.

SHOTGUNS

The determination of firing distances involving shotguns must also be related to test firings performed with the suspect weapon using the same type of ammunition known to be used in the crime. In the absence of a weapon, the muzzle-to-target distance can be estimated by measuring the spread of the discharged shot. With close-range shots varying in distance up to 5 feet, the shot charge enters the target as a concentrated mass, producing a hole somewhat larger than the bore of the barrel. As the distance increases, the pellets progressively separate and spread out. Generally speaking, the spread in the pattern made by a 12-gauge shotgun increases 1 inch for each yard of distance. Thus, a 10-inch pattern would be produced at approximately 10 yards. Of course, this is only a rule of thumb; normally, many variables can affect the shot pattern.

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FIGURE 9-19 (a) A shirt bearing a powder stain, photographed under normal light. (b) An infrared photograph of the same shirt.

Other factors include the barrel length, the size and quantity of the pellets fired, the quantity of powder charge used to propel the pellets, and the choke of the gun under examination. Choke is the degree of constriction placed at the muzzle end of the barrel. The greater the choke, the narrower the shotgun pattern and the faster and farther the pellets will travel.

choke An interior constriction placed at or near the muzzle end of a shotgun’s barrel to control shot dispersion.

POWDER RESIDUES ON GARMENTS

When garments or other evidence relevant to a shooting are received in the crime laboratory, the surfaces of all items are first examined microscopically for gunpowder residue. These particles may be identifiable by their characteristic colors, sizes, and shapes. However, the absence of visual indications does not preclude the possibility that gunpowder residue is present. Sometimes the lack of color contrast between the powder and garment or the presence of heavily encrusted deposits of blood can obscure the visual detection of gunpowder. Often, an infrared photograph of the suspect area overcomes the problem. Such a photograph may enhance the visual color contrast, thus revealing vaporous lead and powder particles deposited around the hole (see Figure 9-19). In other situations, this may not help, and the analyst must use chemical tests to detect gunpowder residues.

Nitrites are one type of chemical product that results from the incomplete combustion of smokeless (nitrocellulose) powder.

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One test method for locating powder residues involves transferring particles embedded on the target surface to chemically treated gelatin-coated photographic paper. This procedure is known as the Greiss test. The examiner presses the photographic paper onto the target with a hot iron; once the nitrite particles are on the paper, they are made easily visible through chemical treatment. In addition, comparing the developed nitrite pattern to nitrite patterns obtained from test firings at known distances can be useful in determining the shooting distance from the target. A second chemical test is then performed to detect any trace of lead residue around the bullet hole. The questioned surface is sprayed with a solution of sodium rhodizonate, followed by a series of oversprays of acid solutions. This treatment turns lead particles pink then blue-violet.

Greiss test A chemical test used to examine patterns of gunpowder residues around bullet holes.

Quick Review

• The distribution of gunpowder particles and other discharge residues around a bullet hole permits an assessment of the distance from which a handgun or rifle was fired. • The precise distance from which a handgun or rifle was fired is deter-mined by carefully comparing the powder residue pattern on the victim’s clothing to test patterns made when the suspect weapon is fired at varying distances from a target. • The Greiss test is a chemical test used to examine patterns of gunpowder

Primer Residues on the Hands

The firing of a weapon not only propels residues toward the target, but it also blows gunpowder and primer residues back toward the shooter (see Figure 9-20). As a result, traces of these residues are often deposited on the firing hand of the shooter, and their detection can provide valuable information about whether an individual has recently fired a weapon.

DETECTING PRIMER RESIDUES

Early efforts at demonstrating powder residues on the hands centered on chemical tests that could detect unburned gunpowder or nitrates. For many years, the dermal nitrate test was popular. It required the application of hot paraffin or wax to the suspect’s hand with a paintbrush. After drying into a solid crust, the paraffin was removed and tested with diphenylamine. A blue color indicated a positive reaction for nitrates. However, the dermal nitrate test has fallen into disfavor with law enforcement agencies, owing mainly to its lack of specificity. Common materials such as fertilizers, cosmetics, urine, and tobacco all give positive reactions that are indistinguishable from that obtained for gunpowder by this test.

Efforts to identify a shooter now center on the detection of primer residues deposited on the hand of a shooter at the time of firing. With the exception of those in most .22-caliber ammunition, primers currently manufactured contain a blend of lead styphnate, barium nitrate, and antimony sulfide. Residues from these materials are most likely to be deposited on the thumb web and the back of the firing hand of a shooter because these areas are closest to gases escaping along the side or back of the gun during discharge. In addition, individuals who handle a gun without firing it may have primer residues deposited on the palm of the hand where it has been in contact with the weapon.

FIGURE 9-20 When a handgun is fired, gunpowder and primer residues are normally blown back toward the hand of the shooter.

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Courtesy Forensic Technology WAI Inc.

However, with the handling of a used firearm, the passage of time, and the resumption of normal activities following a shooting, gunshot residues from the back of the hand are frequently redistributed to other areas, including the palms. Therefore, it is not unusual to find higher levels of barium and antimony on the palms than on the backs of the hands of known shooters. Another possibility is the deposition of significant levels of barium and antimony on the hands of an individual who is near a firearm when it is discharged.

TESTS FOR PRIMER RESIDUES

The determination of whether a person has fired or handled a weapon or has been near a discharged firearm is normally made by measuring the presence and possibly the amount of barium and antimony on the relevant portions of the suspect’s hands. A variety of materials and techniques are used for removing these residues. The most popular approach, and certainly the most convenient for the field investigator, is to apply an adhesive tape or adhesive to the hand’s surface to remove any adhering residue particles.

SWABBING

Another approach is to remove any residues present by swabbing both the firing and nonfiring hands with cotton that has been moistened with 5 percent nitric acid. The front and back of each hand are swabbed separately. All four swabs, along with a moistened control, are then forwarded to the crime laboratory for analysis.

In any case, once the hands are treated for the collection of barium and antimony, the collection medium must be analyzed

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for the presence of these elements. High barium and antimony levels on the suspect’s hand(s) strongly indicate that the person fired or handled a weapon or was near a firearm when it was discharged. Because these elements are normally present in small quantities (e.g., less than 10 micrograms) after a firing, only the most sensitive analytical techniques can be used to detect them.

FIGURE 9-21 An SEM view of gunshot residue particles.

Courtesy Foster and Freeman Limited, Worchester Shine, U.K., www.fosterfreeman.co.uk

Unfortunately, even though most specimens submitted for this type of analysis have been obtained from individuals strongly suspected of having fired a gun, there has been a low rate of positive findings. The major difficulty appears to be the short time that primer residues remain on the hands. These residues are readily removed by intentional or unintentional washing, rubbing, or wiping of the hands. In fact, one study demonstrated that it is very difficult to detect primer residues on cotton

hand swabs taken just two hours after firing a weapon.2 Hence, some laboratories do not accept cotton hand swabs taken from living people six or more hours after a firing has occurred.

In cases that involve suicide victims, a higher rate of positive findings for the presence of gunshot residue is obtained when

the hand swabbing is conducted before the person’s body is moved or when the hands are protected by paper bags.3

However, hand swabbing or the application of an adhesive cannot be used to detect firings of most .22-caliber rim-fire ammunition. Such ammunition’s primer may contain only barium or neither barium nor antimony.

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SEM TESTING

Most laboratories that can detect gunshot residue require application of an adhesive to the shooter’s hands Microscopic primer and gunpowder particles on the adhesive are then found with a scanning electron microscope (SEM). The characteristic size and shape of these particles distinguishes them from other contaminants on the hands (see Figure 9-21). When the SEM is linked to an X-ray analyzer, an elemental analysis of the particles can be conducted. A finding of a select combination of elements (i.e., lead, barium, and antimony) confirms that the particles are indeed primer residue (see Figure 9-22).

The major advantage of the SEM approach for primer residue detection is its enhanced specificity over hand swabbing. The SEM characterizes primer particles by their size and shape as well as by their chemical composition. Unfortunately, the excessive operator time required to find and characterize gunshot residue has discouraged the use of this technique. The availability of automated particle search and identification systems for use with scanning electron microscopes may overcome this problem. Results of work performed with automated systems show that it is significantly faster than a manual

approach for finding gunshot residue particles.4 Appendix II contains a detailed description of primer residue collection procedures.

Quick Review

• Firing a weapon propels residues toward the target and blows gunpowder and primer residues back toward the shooter. Traces of these residues are often deposited on the firing hand of the shooter, providing valuable information about whether an individual has recently fired a weapon.

FIGURE 9-22 A spectrum showing the presence of lead, barium, and antimony in gunshot residue.

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• Examiners measure the amount of barium and antimony on the relevant portion of the suspect’s hands or characterize the morphology of particles containing these elements to determine whether a person has fired or handled a weapon or was near a discharged firearm.

Serial Number Restoration

Today, many manufactured items, including automobile engine blocks and firearms, are impressed with serial numbers for identification. Increasingly, the criminalist must restore such numbers when they have been removed or obliterated by grinding, rifling, or punching.

Serial numbers are usually stamped, on a metal body or frame or on a metal plate, with hard steel dies. These dies strike the metal surface with a force that allows each digit to sink into the metal at a prescribed depth. Serial numbers can be restored because the metal crystals in the stamped zone are placed under a permanent strain that extends a bit beneath the original numbers. When a suitable etching agent is applied, the strained area dissolves faster than the unaltered metal, thus revealing

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the etched pattern in the form of the original numbers (see Figure 9-23). However, if the zone of strain has been removed, or if the area has been impressed with a different strain pattern, the number usually cannot be restored.

Before any treatment with the etching reagent, the obliterated surface must be thoroughly cleaned of dirt and oil and polished to a mirror like finish. The reagent is swabbed onto the surface with a cotton ball. The choice of etching reagent depends on the type of metal surface being worked on. A solution of hydrochloric acid (120 milliliters), copper chloride (90 grams), and water (100 milliliters) generally works well for steel surfaces.

CLOSER ANALYSIS THE SCANNING ELECTRON MICROSCOPE (SEM)

The scanning electron microscope (SEM) creates an image by aiming a beam of electrons at a specimen, then the electron emissions from the specimen are studied on a closed-circuit TV (see Figure 1). This is accomplished by using electromagnetic focusing to direct electrons emitted by a hot tungsten filament onto the surface of the specimen. This primary electron beam causes the elements that make up the upper layers of the specimen to emit electrons known as secondary electrons. About 20 to 30 percent of the primary electrons rebound off the surface of the specimen. These electrons are known as backscattered electrons. The emitted electrons (both secondary and backscattered) are collected, and the amplified signal is displayed on a cathode-ray, or TV, tube. By scanning the primary electron beam across the specimen’s surface in synchronization with the cathode-ray tube, the SEM converts the emitted electrons into an image of the specimen that displays on the cathode-ray tube.

The major attractions of the SEM image are its high magnification, high resolution, and great depth of focus. In its usual mode, the SEM has a magnification that ranges from 10× to 100,000×. Its depth of focus is 300 times better than that of optical systems at similar magnifications, and the resultant picture is almost stereoscopic in appearance. Its great depth of field and magnification are exemplified in the magnified cystolithic hair on the marijuana leaf shown in Figure 2. A SEM image of a vehicle’s headlight filaments may reveal whether the headlights were on or off at the time of a collision (see Figures 3 and 4).

FIGURE 1 A scanning electron microscope.

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FIGURE 2 The cystolithic hairs of the marijuana leaf, as viewed with a scanning electron microscope (800x).

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FIGURE 3 The melted ends of a hot filament break indicate that the headlights were on when an accident occurred.

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Another facet of scanning electron microscopy is the use of X-ray production to determine the elemental composition of a specimen. X-rays are generated when the electron beam of the scanning electron microscope strikes a target. When the SEM is coupled with an X-ray analyzer, the emitted X-rays can be sorted according to their energy values and used to build a picture of the elemental distribution in the specimen. Because each element emits X-rays of characteristic energy values, the X-ray analyzer can identify the elements present in a specimen. Furthermore, the elemental concentration can be determined by measuring the intensity of the X-ray emission.

As shown in Figure 5, when a sample of gunshot residue collected off the hands of a suspect shooter is exposed to a beam of electrons from the scanning electron microscope, X-rays are emitted. These X-rays are passed into a detector, where they are converted into electrical signals. These signals are sorted and displayed according to the energies of the emitted X-rays. Through the use of this technique, the elements lead, antimony, and barium, frequently found in most primers, can be rapidly detected and identified.

FIGURE 4 The sharp ends of a cold filament break indicate that the headlights were off when an accident occurred.

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FIGURE 5 A schematic diagram of a scanning electron microscope displaying the image of a gunshot residue particle. Simultaneously, an X-ray analyzer detects and displays X-ray emissions from the elements lead (Pb), antimony (Sb), and barium (Ba) present in the particle.

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FIGURE 9-23 Obliterated or altered serial numbers on firearms can be restored by analysts using chemical means.

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Federal Bureau of Investigation

Collection and Preservation of Firearms Evidence

FIREARMS

The Hollywood technique of picking up a weapon by its barrel with a pencil or stick in order to protect fingerprints must be avoided. This practice only disturbs powder deposits, rust, or dirt lodged in the barrel, and consequently may alter the striation markings on test-fired bullets. If recovery of latent fingerprints is a primary concern, the investigator should hold the weapon by the edge of the trigger guard or by the checkered portion of the grip, which usually does not retain identifiable fingerprints.

The most important consideration in handling a weapon is safety. Before any weapon is sent to the laboratory, all precautions must be taken to prevent an accidental discharge of a loaded weapon in transit. In most cases, it will be necessary to unload the weapon. If this is done, first a record should be made of the weapon’s hammer and safety position; likewise, the location of all fired and unfired ammunition in the weapon must be recorded.

When a revolver is recovered, the chamber position should be indicated by a scratch mark on the cylinder where it aligns with the barrel. Each chamber is designated a number on a diagram, and as each cartridge or casing is removed, it should be marked to correspond to the number of its corresponding chamber in the diagram. Knowledge of the cylinder position of a cartridge casing may be useful for later determination of the sequence of events, particularly in shooting cases, when more

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than one shot was fired. Each round should be placed in a separate box or envelope. If the weapon is an automatic, the magazine must be removed and checked for prints and the chamber then emptied.

As with any other type of physical evidence recovered at a crime scene, firearms evidence must be marked for identification, and a chain of custody must be established. When a firearm is recovered, an identification tag should be attached to the trigger guard. The tag should include appropriate identifying data, including the weapon’s serial number, make, and model and the investigator’s initials.

When a weapon is recovered from an underwater location, no effort should be made to dry or clean it. Instead, the firearm should be transported to the laboratory in a receptacle containing enough of the same water to keep it submerged. This procedure prevents rust from developing during transport.

AMMUNITION

The protection of class and individual markings on bullets and cartridge cases must be the primary concern of the field investigator. Thus, extreme caution is needed when removing a lodged bullet from a wall or other object. If the bullet’s surface is accidentally scratched during this operation, valuable striation markings could be obliterated. It is best to free bullets from their target by carefully breaking away the surrounding support material while avoiding direct contact with the projectile.

Bullets, cartridge casings, and discharged shells from shotguns should just be placed in a container that is appropriately marked for identification. It is recommended that the investigator not directly mark these items with a scribe. In any case, the investigator must protect the bullet by wrapping it in tissue paper before placing it in a pillbox or an evidence envelope for shipment to the crime laboratory. Minute traces of evidence such as paint and fibers may be adhering to the bullet; the investigator must take care to leave these trace materials intact.

When semiautomatic or automatic weapons have been fired, the ejection pattern of the casings can help establish the relationship of the suspect to the victim. For this reason, the investigator must note the exact location where a shell casing was recovered.

GUNPOWDER DEPOSITS

The clothing of a firearms victim must be carefully preserved to prevent damage or disruption to powder residues deposited around a bullet or shot hole. Cutting or tearing of clothing in the area of the holes must be avoided when removing the clothing. All wet clothing should be air-dried out of direct sunlight and then folded carefully to avoid disrupting the area around the bullet hole. Each item should be placed in a separate paper bag.

Quick Review

• Criminalists can restore serial numbers removed or obliterated by grinding, rifling, or punching. • Because the metal crystals in the stamped zone are placed under a permanent strain that extends a bit beneath the original numbers, the serial number can sometimes be restored through chemical etching. • A suspect firearm should never be picked up by inserting an object into its barrel because this practice may alter the striation markings on test-fired bullets. • Before unloading a suspect weapon, the weapon’s hammer and safety position should be recorded, as well as the locations of all fired and unfired ammunition in the weapon. • The protection of class and individual markings on bullets and cartridge cases is the primary concern of the field investigator when recovering bullets and cartridge casings.

Tool Marks

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A tool mark is any impression, cut, gouge, or abrasion caused by a tool coming into contact with another object. Most often, tool marks are encountered at burglary scenes that involve forcible entry into a building or safe. Generally, these marks occur as indented impressions into a softer surface or as abrasion marks caused by the tool cutting or sliding against another object.

COMPARING TOOL MARKS

Typically, an indented impression is left on the frame of a door or window as a result of the prying action of a screwdriver or crowbar. Careful examination of these impressions can reveal important class characteristics—that is, the size and shape of the tool. However, they rarely reveal any significant individual characteristics that could permit the examiner to individualize the mark to a single tool. Such characteristics, when they do exist, usually take the form of discernible random nicks and breaks that the tool has acquired through wear and use (see Figure 9-24).

Just as the machined surfaces of a firearm are impressed with random striations during its manufacture, the edges of a pry bar, chisel, screwdriver, knife, or cutting tool likewise display a series of microscopic irregularities that look like ridges and valleys. Such markings are created as a result of the machining processes used to cut and finish tools. The shape and pattern of such minute imperfections are further modified by damage and wear during the life of the tool. Considering the variety of patterns that the hills and valleys can assume, it is highly unlikely that any two tools will be identical. Hence, these minute imperfections impart individuality to each tool.

FIGURE 9-24 A comparison of a tool mark with a suspect screwdriver. Note how the presence of nicks and breaks on the tool’s edge helps individualize the tool to the mark.

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If the edge of a tool is scraped against a softer surface, it may cut a series of striated lines that reflect the pattern of the tool’s edge. With the aid of a comparison microscope, markings left in this manner can be compared in the laboratory with test tool marks made from the suspect tool. When a sufficient number of striations match between the evidence and test markings, the result can be a positive comparison, and hence a definitive association of the tool with the evidence mark.

A major problem of tool mark comparisons is the difficulty in duplicating in the laboratory the tool mark left at the crime scene. A thorough comparison requires preparing a series of test marks by applying the suspect tool at various angles and pressures to a soft metal surface (lead is commonly used). This approach gives the examiner ample opportunities to duplicate many of the details of the original evidence marking. A photomicrograph of a typical tool mark comparison is illustrated in Figure 9-25.

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FIGURE 9-25 A photograph of a tool mark comparison seen under a comparison microscope.

Courtesy Leica Microsystems, Buffalo, NY, www.leica-microsystems.com

FIGURE 9-26 (a) Casting a tool mark impression with a silicone-based putty. (b) An impression alongside a suspect tool.

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Courtesy Sirchie Finger Print Laboratories, Inc., Youngsville, NC, www.sirchie.com

COLLECTING TOOL MARK EVIDENCE

Whenever practical, the entire object or the part of the object bearing a tool mark should be submitted to the crime laboratory for examination. When removal of the tool mark is impractical, the only recourse is to photograph the marked area to scale and then make a cast of the mark. Liquid silicone casting material is best for reproducing most of the fine details of a mark (see Figure 9-26). However, even under the best conditions, the clarity of many of the tool mark’s minute details will be lost or obscured in a photograph or cast. Of course, this will reduce the chance of individualizing the mark to a single tool.

The crime-scene investigator must never attempt to fit the suspect tool into the tool mark. Any contact between the tool and the marked surface may alter the mark and will, at the very least, raise serious questions about the integrity of the evidence. The suspect tool and mark must be packaged in separate containers, and every precaution must be taken to avoid contact between the tool and mark and another hard surface. Failure to protect the tool and mark from damage could result in the destruction of their individual characteristics.

Furthermore, the tool or its impression may contain valuable trace evidence. Chips of paint adhering to the mark or tool provide perhaps the best example of how the transfer of trace physical evidence can occur as a result of using a tool to gain forcible entry into a building. Obviously, the presence of trace evidence greatly enhances the evidential value of a tool or its mark. Preserving such evidence requires special care in handling and packaging to avoid loss or destruction.

Quick Review

• The presence of minute imperfections on a tool imparts individuality to that tool. The shape and pattern of such imperfections are further modified by damage and wear during the life of the tool.

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• The comparison microscope is used to compare crime-scene tool marks with test impressions made with the suspect tool.

Other Impressions

From time to time, other types of impressions are left at a crime scene. This evidence may take the form of a shoe, tire, or fabric impression. It may be as varied as a shoe impression left on a piece of paper at the scene of a burglary (Figure 9-27), a hit-and-run victim’s garment that has come into violent contact with an automobile (Figure 9-28), or the impression of a bloody shoe print left on a floor or carpet at a homicide scene (Figure 9-29).

PRESERVING IMPRESSIONS

The primary consideration in collecting impressions at the crime scene is the preservation of the impression or its reproduction for later examination in the crime laboratory. Before any impression is moved or otherwise handled, it must be photographed to show all the observable details of the impression (a scale should be included in the picture). Several shots should be taken directly over the impression as well as at various angles around the impression. Skillful use of side lighting for illumination will help highlight many ridge details that might otherwise remain obscured. Photographs should also be taken to show the position of the questioned impression in relation to the overall crime scene.

Although photography is an important first step in preserving an impression, it must be considered merely a backup procedure that is available to the examiner should the impression be damaged before it reaches the crime laboratory. Naturally, the examiner prefers to receive the original impression to compare to the suspect shoe, tire, garment, and so forth. In most cases, when the impression is on a readily recoverable item, such as glass, paper, or floor tile, the evidence is easily transported intact to the laboratory.

FIGURE 9-27 (a) An impression of a shoe found at a crime scene. (b) A test impression made with a suspect shoe. A sufficient number of points of comparison exist to support the conclusion that the suspect shoe left the impression at the crime scene.

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FIGURE 9-28 A small child was found dead at the edge of a rural road near a railroad crossing, the victim of a hit-and-run driver. A local resident was suspected, but he denied any knowledge of the incident. The investigating officer noted what appeared to be a fabric imprint on the bumper of the suspect’s automobile. The weave pattern of the clothing of the deceased was compared with the imprint on the bumper and was found to match. When the suspect was confronted with this information, he admitted his guilt.

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Courtesy Centre for Forensic Sciences, Toronto, Canada

FIGURE 9-29 A bloody imprint of a shoe was found on the carpet in the home of a homicide victim. (b) The suspect’s shoe, shown in (a), made the impression. Note the distinctive impression of the hole present in the shoe’s sole.

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Courtesy Dade County Crime Lab

LIFTING IMPRESSIONS

If an impression on a surface that cannot be submitted to the laboratory is encountered, the investigator may be able to preserve the print in a manner that is analogous to lifting a fingerprint. This is especially true of impressions made in light deposits of dust or dirt. A lifting material large enough to lift the entire impression should be used. Carefully place the lifting material over the entire impression. Use a fingerprint roller to eliminate any air pockets before lifting the impression off the surface.

A more exotic approach to lifting and preserving dust impressions involves the use of a portable electrostatic lifting device. The principle is similar to that of creating an electrostatic charge on a comb and using the comb to lift small pieces of tissue paper. A sheet of Mylar film is placed on top of the dust mark, and the film is pressed against the impression with the aid of a roller. The high-voltage electrode of the electrostatic unit is then placed in contact with the film while the unit’s earth electrodes are placed against a metal plate, or earth plate (see Figure 9-30). A charge difference develops between the Mylar film and the surface below the dust mark, so the dust attaches to the lifting film. In this manner, dust prints on chairs, walls, floors, and the like, can be transferred to Mylar film. Floor surfaces up to 40 feet long can be covered with a Mylar sheet and searched for dust impressions. The electrostatic lifting technique is particularly helpful in recovering barely visible dust prints on colored surfaces. Dust impressions can also be enhanced through chemical development (see Figure 9-31).

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WebExtra 9.7

Casting a Footwear Impression www.mycrimekit.com

CASTING IMPRESSIONS

Shoe and tire marks impressed into soft earth at a crime scene are best preserved by photography and casting. Class I dental stone, a form of gypsum, is widely recommended for making casts of shoe and tire impressions. The cast should be allowed to air-dry for 24 to 48 hours before it is shipped to the forensic science laboratory for examination. Figure 9-32 illustrates a cast made from a shoe print in mud. The cast compares to the suspect shoe.

FIGURE 9-30 Electrostatic lifting of a dust impression off a floor using an electrostatic unit.

Courtesy Sirchie Finger Print Laboratories, Inc., Youngsville, NC, www.sirchie.com

FIGURE 9-31 (a) A dust impression of a shoe print on cardboard before enhancement. (b) A shoe print after chemical enhancement with bromophenol blue and exposure to water vapor.

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Courtesy Division of Identification and Forensic Science, Israel Police Headquarters

FIGURE 9-32 (a) A shoe impression in mud. (b) A cast of a shoe impression. (c) A shoe suspected of leaving the muddy impression.

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Courtesy Sirchie Fingerprint Laboratories, Inc., Youngsville, NC, www.sirchie.com

An aerosol product known as Snow Impression Wax is available for casting snow impressions. The recommended procedure is to spray three light coats of the wax at an interval of one to two minutes between layers, and then let it dry for ten minutes. A viscous mixture of Class I dental stone is then poured into the wax-coated impression. After the casting material has hardened, the cast can be removed.

Several chemicals can be used to develop and enhance footwear impressions made with blood. In areas where a bloody footwear impression is very faint or where a subject has tracked through blood, leaving a trail of bloody impressions, chemical enhancement can visualize latent or nearly invisible footwear impressions (see Figure 9-33). A number of chemical formulas useful for bloody footwear impression analysis are listed in Appendix IV.

Several blood enhancement chemicals have been examined for their impact on short tandem repeat (STR) DNA typing. (This particular method of DNA analysis will be discussed in Chapter 15.) None of the chemicals examined had a deleterious

effect, on a short-term basis, on the ability to carry out STR DNA typing on the blood.5

COMPARING IMPRESSIONS

Whatever the circumstances, the laboratory procedures used to examine any type of impression remain the same. Of course, a comparison is possible only when the item suspected of having made the impression is recovered. Test impressions may be necessary to compare the characteristics of the suspect item with the evidence impression.

The evidential value of the impression is determined by the number of class and individual characteristics that the examiner

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finds. Agreement with respect to size, shape, or design may permit the conclusion that the impression could have been made by a particular shoe, tire, or garment, but one cannot entirely exclude other possible sources from having the same class characteristics. More significant is the existence of individual characteristics arising out of wear, cuts, gouges, or other damage. A sufficient number or the uniqueness of such points of comparison supports a finding that both the evidence and test impressions originated from only one source.

FIGURE 9-33 (a) A bloody footprint on cardboard treated with amido black. (b) A bloody footprint treated with Hungarian red dye. (c) A bloody footprint visualized with leucocrystal violet. (d) A bloody footprint enhanced with patent blue.

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(a) Courtesy Dwane S. Hilderbrand and David P. Coy, Scottsdale Police Crime Laboratory, Scottsdale, AZ; (b) Courtesy ODV Inc., South Plains, Maine;

(c–d) Courtesy William Bodziak, FBI Laboratory

CLOSER ANALYSIS CASTING FOOTWEAR AND TIRE IMPRESSIONS

Footwear and tire impressions may be found at any type of crime scene and can provide a primary means to identify or exclude a suspect. The preferred method of collection for this type of evidence is casting the impression—that is, making a mold and preserving it for analysis in the lab. When a footwear or tire impression is found in dirt at the crime scene, the casting process is as follows:

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MATERIALS Ruler One small can of aerosol hair spray 1-gallon zip-top bag Paint stirrer or large, long-handled spoon Carton of dental stone Water Camera Plastic or metal casting frame (optional)

Procedure

1. Retrieve any fragments or debris that is not imbedded within the impression. Photograph the impression before and after retrieving debris; include a ruler in the photograph. A frame for containing the dental stone may be installed around an impression that is shallow or located on an inclined surface. 2. To solidify the soil, a fixative such as hair spray is used (see [a]). Hold the can of hair spray about 18 inches from the soil within the impression. Very lightly, spray an even layer to the impression using a sweeping motion and taking care to avoid any damage to the impression. 3. Wait ten minutes to allow the hair spray to dry. 4. Add an appropriate amount of water to a premeasured amount of dental stone (see [b]). Add water in increments. The usual amount is about 10 to 12 fluid ounces of water to about 1.5 to 2 pounds of dental stone. If using a zip-top bag, seal the bag and mix by working back and forth with your fingers for at least three minutes (see [c]). Mix until a pancake-batter-like consistency is reached.

Casting a footwear impression at a crime scene: (a) The impression is hardened using aerosol hair spray. (b) The correct amount of water is added to a known amount of dental stone. (c) The mixture is kneaded by hand until the desired (pancake-batter-like) consistency is reached. (d) The dental stone is poured into the impression using a spoon as a medium to disperse the flow. (e) The impression is filled with dental stone and allowed to dry before removal.

Courtesy Sirchie Fingerprint Laboratories, Inc., Youngsville, NC, www.sirchie.com

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5. Open one corner of the bag. Pour the dental stone through the opening onto the ground beside the impression and allow it to carefully run into the impression. Use a paint stirrer, a spoon, or a gloved hand as a medium to disperse the stream so it does not destroy the fine details of the impression (see [d]). Continue pouring until the dental stone completely fills the impression (see [e]) and reaches at least 1/2 inch in thickness. If necessary, additional casting material may be poured over the top of the original cast to add thickness. 6. Label the wet plaster surface with the date, initials, and any other information required for evidence labeling. 7. When the cast no longer adheres to the soil and is relatively dry (usually about one hour), remove the cast. If necessary, the cast can be dug out from the sides. 8. Store the cast for 48 hours to allow it to dry completely. If a cast is not allowed to dry long enough, some ridge details may disappear. 9. Once the cast is dry, rinse any loose soil from it with softly running water. A soft-bristled brush may also be used. Do not scrub or pick off anything. Pat dry with paper towels.

When a tire tread impression is left at a crime scene, the laboratory can examine the design of the impression and possibly determine the style and/or manufacturer of the tire. This may be particularly helpful to investigators when a suspect tire has not yet been located.

FIGURE 9-34 (a) A bite mark impression on the victim’s forearm. (b) An upper dental model from the teeth of the suspect matches the individual tooth characteristics of the bite marks.

Courtesy the late Haskin Askin, D.D.S., Chief Forensic Odontologist, Brick Town, NJ 08724

New computer software may help the forensic scientist compare shoe prints. For example, an automated shoe print identification system developed in England, called shoeprint image capture and retrieval (SICAR), incorporates multiple databases to search known and unknown footwear files for comparison against footwear specimens. With the system, an impression from a crime scene can be compared to a reference database to find out what type of shoe caused the imprint. That same impression can also be searched in the suspect and crime databases to reveal whether that shoe print matches the shoes of a person who has been in custody or the shoe prints left behind at another crime scene. When matches are made during the searching process, the images are displayed side by side on the computer screen (see Figure 5-9 in Chapter 5).

CASE FILES THE O. J. SIMPSON TRIAL: WHO LEFT THE

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IMPRESSIONS AT THE CRIME SCENE?

On the night of June 12, 1994, Nicole Brown—ex-wife of football star O. J. Simpson—and her friend Ron Goldman were brutally murdered on the grounds outside her home in Brentwood, California. O. J. Simpson was arrested for their murders but professed his innocence. At the crime scene, investigators found bloody shoe impressions along the concrete walkway leading up to the front door of Brown’s condominium. These shoe impressions were of extremely high quality and of intricate detail. The news media broadcast countless images of these bloody shoe prints on television, making it obvious to the killer that those shoes would surely link him to the crime.

Famed FBI shoe print examiner William J. Bodziak investigated the footwear evidence from the scene. His first task was to identify the brand of shoe that made the marks. Because the pattern was very clear and distinct, with complete toe-to-heel detail, this seemed like a simple task at first. Bodziak compared this pattern to the thousands of sole patterns in the FBI’s database. None matched. He then went to his reference collection of books and trade show brochures, again with no success.

Bodziak’s experience told him that these were expensive, Italian-made casual dress shoes with a sole made from synthetic material. Using this knowledge, he shopped the high-end stores for a similar tread pattern but still was unable to identify the shoes. He then drew a composite sketch of the sole and faxed the image to law enforcement agencies and shoe manufacturers and distributors worldwide. The owner of the American distributing company for Bruno Magli shoes was the only one to respond.

Further exhaustive investigation revealed that these were extremely rare shoes. There were two styles of shoe bearing this exact sole design. They had been available for only two years at a mere forty stores in the United States and Puerto Rico. The Lorenzo style had a bootlike upper that came to the ankle. The Lyon style had a lower, more typical dress shoe shape. The impressions had been made by a size 12 shoe, and it was later determined that only 299 pairs of size 12 with this tread pattern were sold in the United States.

Simpson flatly denied ever owning these shoes, adding that he would never wear anything so ugly. However, he was known to wear a size 12, and photographs taken almost nine months before the murders show Simpson wearing a pair of black leather Bruno Magli Lorenzo shoes. These shoes were available in several colors, so this narrows the number of shoes matching Simpson’s pair of Lorenzos (this size, color, and style) sold in the United States to twenty-nine pairs.

Proving that Simpson owned a pair of shoes that had the exact pattern found printed in blood at the crime scene was an essential component of the case, but it was not done in time to be used during the criminal prosecution. The photographs of Simpson in his Bruno Magli shoes were released after the culmination of the criminal trial, so the jury never heard the direct evidence that Simpson owned these shoes. However, this proved to be an important link uniting Simpson with the crime scene in the civil trial. Although O. J. Simpson was acquitted of the murders of Nicole Brown and Ron Goldman in the criminal trial, he was judged responsible for their murders in the civil court case.

Virtual Lab Footwear Impressions

To perform a virtual footwear impression analysis, go to www.pearsoncustom.com/us/vlm/

Virtual Lab Tool Mark Analysis

To perform a virtual tool mark analysis, go to www.pearsoncustom.com/us/vlm/

Human bite marks on skin and foodstuffs have been important items of evidence for convicting defendants in a number of homicide and rape cases in recent years. If a sufficient number of points of similarity between test and suspect bite marks are present, a forensic odontologist may conclude that a bite mark was made by a particular individual (see Figure 9-34).

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Quick Review

• Shoe and tire marks impressed into soft earth at a crime scene are best preserved by photography and casting. • The electrostatic lifting technique is particularly helpful in recovering barely visible dust prints on floor surfaces. • In areas where a bloody footwear impression is very faint or where the subject has tracked through blood and left a trail of bloody impressions, chemical enhancement can visualize latent or nearly invisible blood impressions.

CHAPTER REVIEW

• The manufacture of a gun barrel requires impressing its inner surface with spiral grooves, a step known as rifling. Rifling imparts spin to the projectile when it is fired, which keeps it on an accurate course. • No two rifled barrels have identical striation markings. These striations form the individual characteristics of the barrel. The inner surface of the barrel of a gun leaves its striation markings on a bullet passing through it. • The class characteristics of a rifled barrel include the number of lands and grooves and the width and direction of twist. • The comparison microscope is a firearms examiner’s most important tool because it allows two bullets to be compared simultaneously. • The firing pin, breech face mark, and ejector and extractor mechanism also offer a highly distinctive signature for individualization of cartridge cases. • Unlike handguns, a shotgun is not rifled—it has a smooth barrel. Because of this, shotgun shells are not impressed with any characteristic rifling striation markings that can be used to compare two shotgun shells to determine whether they were fired from the same weapon. • The advent of computerized imaging technology has made possible the storage of bullet and cartridge surface characteristics in a manner analogous to automated fingerprint files. • Two automated firearms search systems are DRUGFIRE, developed by the FBI, and IBIS, developed by the ATF. • NIBIN is the National Integrated Ballistics Information Net-work, a unified firearms search system that incorporates both DRUGFIRE and IBIS technologies. • The distribution of gunpowder particles and other discharge residues around a bullet hole permits an assessment of the distance from which a handgun or rifle was fired. • The precise distance from which a handgun or rifle was fired is determined by carefully comparing the powder residue pattern on the victim’s clothing to test patterns made when the suspect weapon is fired at varying distances from a target. • The Greiss test is a chemical test used to examine patterns of gunpowder residues around bullet holes. It tests for the presence of nitrates. • Firing a weapon propels residues toward the target and blows gunpowder and primer residues back toward the shooter. Traces of these residues are often deposited on the firing hand of the shooter, providing valuable information about whether an individual has recently fired a weapon. • Examiners measure the amount of barium and antimony on the relevant portion of the suspect’s hands or characterize the morphology of particles containing these elements to determine whether a person has fired or handled a weapon or was near a discharged firearm. • Criminalists can restore serial numbers removed or obliterated by grinding, rifling, or punching. • Because the metal crystals in the stamped zone are placed under a permanent strain that extends a short distance beneath the original numbers, the serial number can be restored through chemical etching. • A suspect firearm should never be picked up by inserting an object into its barrel because this practice may alter the striation markings on test-fired bullets. • Before unloading a suspect weapon, the weapon’s hammer and safety position should be recorded, as well as the location of all fired and unfired ammunition in the weapon. • The protection of class and individual markings on bullets and cartridge cases is the primary concern of the field investigator when recovering bullets and cartridge casings. • The presence of minute imperfections on a tool imparts individuality to that tool. The shape and pattern of such

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imperfections are further modified by damage and wear during the life of the tool. • The comparison microscope is used to compare crime-scene tool marks with test impressions made with the suspect tool. • Shoe and tire marks impressed into soft earth at a crime scene are best preserved by photography and casting. • The electrostatic lifting technique is particularly helpful in recovering barely visible dust prints on floor surfaces. • In areas where a bloody footwear impression is very faint or where the subject has tracked through blood and left a trail of bloody impressions, chemical enhancement can visualize latent or nearly invisible blood impressions.

KEY TERMS

bore, 192

breech face mark, 197

caliber, 192

choke, 205

distance determination, 203

ejector, 198

extractor, 198

firearms identification, 190

gauge, 197

Greiss test, 206

grooves, 192

lands, 192

rifling, 192

REVIEW QUESTIONS

Firearms can be divided into two categories: ______________ and ______________ guns.

Handguns, or pistols, are firearms that are designed to be held and fired with one hand, and the most common types of handguns are ______________, ______________, and______________.

The ______________ features several firing chambers, eachholding one cartridge, located within a revolving cylinder that lines the chamber up with the barrel mechanically when the round is fired.

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A cartridge for a shotgun, called a shell, contains numerous ball-shaped projectiles, called ______________.

A shotgun barrel is not rifled and can also be narrowed toward the muzzle in order to concentrate shot when fired. The degree of narrowing of the barrel is called the ______________ of the shotgun.

The ______________ is the original part of the bore left after rifling grooves are formed.

The diameter of the gun barrel is known as its ______________.

True or False: The number of lands and grooves is a class characteristic of a barrel. ______________

The ______________ characteristics of a rifled barrel are formed by striations impressed into the barrel’s surface.

10.

The most important instrument for comparing bullets is the ______________.

11.

To make a match between a test bullet and a recovered bullet, the lands and grooves of the test and evidence bullet must have identical widths, and the longitudinal ______________ on each must coincide.

12.

True or False: It is always possible to determine the make of a weapon by examining a bullet it fired. ______________

13.

A shotgun has a(n) ______________ barrel.

14.

The diameter of a shotgun barrel is expressed by the term ______________.

15.

True or False: Shotgun pellets can be individualized to a single weapon. ______________

16.

True or False: A cartridge case can be individualized to a single weapon. ______________

17.

The automated firearms search system developed by the FBI and ATF as a unified system incorporating both DRUGFIRE and IBIS technologies is known as ______________.

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18.

True or False: The distribution of gunpowder particles and other discharge residues around a bullet hole permits an approximate determination of the distance from which the gun was fired. ______________

19.

True or False: Without the benefit of a weapon, an examiner can make an exact determination of firing distance. ______________

20.

A halo of vaporous lead deposited around a bullet hole normally indicates a discharge ______________ to______________ inches from the target.

21.

If a firearm has been fired more than 3 feet from a target, usually no residue is deposited, but a dark ring, known as ______________, is observed.

22.

As a rule of thumb, the spread in the pattern made by a 12-gauge shotgun increases 1 inch for every ______________ of distance from the target.

23.

A(n) ______________ photograph may help visualize gunpowder deposits around a target.

24.

True or False: One test method for locating powder residues involves transferring particles embedded on the target surface to chemically treated photographic paper. ______________

25.

Current methods for identifying a shooter rely on the detection of ______________ residues on the hands.

26.

Determining whether an individual has fired a weapon is done by measuring the elements ______________ and______________ present on the hands.

27.

True or False: Firings with all types of ammunition can be detected from hand swabbings with nitric acid. ______________

28.

Microscopic primer and gunpowder particles on the adhesives applied to a suspected shooter’s hand can be detected with a(n) ______________.

29.

True or False: Restoration of serial numbers is possible because in the stamped zone the metal is placed under a permanent

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strain that extends beneath the original numbers. ______________

30.

True or False: It is proper to insert a pencil into the barrel when picking up a crime-scene gun. ______________

31.

Recovered bullets are initialed on either the ______________ or ______________ of the bullet.

32.

True or False: Because minute traces of evidence such as paint and fibers may be adhering to a recovered bullet, the investigator must take care to remove these trace materials immediately. ______________

33.

True or False: Cartridge cases are best marked at the base of the shell. ______________

34.

The clothing of the victim of a shooting must be handled so to prevent disruption of ______________ around bullet holes.

35.

A(n) ______________ is any impression caused by a tool coming into contact with another object.

36.

Tool marks compare only when a sufficient number of ______________ match between the evidence and test markings.

37.

Objects bearing tool marks either should be submitted intact to the crime lab, or a(n) ______________ should be taken of the tool mark.

38.

An imprint may be lifted using lifting sheets or a(n) ______________.

39.

Shoe and tire marks impressed into soft earth at a crime scene are best preserved by ______________ and______________.

40.

A wear pattern, cut, gouge, or other damage pattern can impart ______________ characteristics to a shoe.

APPLICATION AND CRITICAL THINKING

Name and briefly describe two popular approaches for collecting gunshot residue from a suspect’s hands. What is the most specific method of analysis for gunshot residue?

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You are investigating a shooting involving a 12-gauge shotgun with a moderately high choke. The spread of the pattern made by the pellets measures 12 inches. In your opinion, which of the following is probably closest to the distance from the target to the shooter? Explain your answer and explain why the other answers are likely to be incorrect.

a) 18 yards b) 12 yards c) 6 yards d) 30 yards

Criminalist Ben Baldanza is collecting evidence from the scene of a shooting. After locating the revolver suspected of firing the shots, Ben picks the gun up by the grip, unloads it, and places the ammunition in an envelope. He then attaches an identification tag to the grip. Searching the scene, Ben finds a bullet lodged in the wall. He uses pliers to grab the bullet and pull it from the wall, then inscribes the bullet with his initials and places it in an envelope. What mistakes, if any, did Ben make in collecting this evidence?

How would you go about collecting impressions in each of the following situations?

a) You discover a shoe print in dry sand. b) You discover a tool mark on a windowsill. c) You discover tire marks in soft earth. d) You discover a shoe print on a loose piece of tile. e) You discover a very faint shoe print in dust on a colored linoleum floor.

Gunshot residue patterns (A) through (D) (contact, 1 inch, 6 inches, and 18 inches) from a 40-caliber pistol are shown below. Match the firing distance to each pattern.

(A) ______________

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(B) ______________

(D) ______________

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