READ: Arson, Firearms and Toolmarks

Introduction

Introduction

Firearms and tools are manufactured and used in such a way that subtle variations can exist. These subtle differences enable investigators to link weapons and tools to a particular crime and sometimes even a suspect. Investigators can also study the trajectory of a projectile to gain more insight into the way in which the crime occurred. While arson fires and explosives are both destructive, they can still leave behind valuable evidence. This evidence can be used to reconstruct the crime as well as trace and connect materials of evidence to the suspect. Tool marks are often left in crimes such as burglary when a suspect attempts to gain entry into a secured location. These tool marks can indicate not only the type of tool used, but also a specific weapon if it has surface irregularities that can be matched to the observed tool markings.

Essential Questions

1. How can a weapon be linked to a crime or individual?

2. How are tool marks analyzed?

3. What do investigators look for to determine if a fire was accidental or arson?

4. How are explosives classified?

Module Minute

Firearms, toolmarks and arson can yield significant forensic evidence to the observant investigator. Often the evidence is subtle and requires experts within that area of specialty to adequately interpret the results of testing. Bullet casings found at a crime scene, for example, can be linked to an individual gun by comparing the firing pin marks and marks from the gun's rifling on the cartridges using a comparison microscope. At a fire scene, fire patterns can be linked to various accelerants which can distinguish an accidental or natural fire from an arson. Toolmarks can be cast just as other prints such as shoe prints can be cast at the crime scene. Often, a silicone based material is used to obtain an exact casting of the toolmarks. That casting can then be compared to known marks of various tools to identify the tool used.

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Arson and Explosives Evidence

Arson and Explosives Evidence

In this module, you will learn about arson, explosives, firearms and tool marks. Of those, arson, explosives and firearms are under the jurisdiction of the ATF, or Bureau of Alcohol, Tobacco, Firearms and Explosives. The mission statement of the ATF is as follows:

"ATF is a unique law enforcement agency in the United State Department of Justice that protects our communities from violent criminals, criminal organizations, the illegal use and trafficking of firearms, the illegal use and storage of explosives, acts of arson and bombings, acts of terrorism, and the illegal diversion of alcohol and tobacco products. We partner with communities, industries, law enforcement, and public safety agencies to safeguard the public we serve through information sharing, training, research and use of technology."

 
Chemistry of Fire Interactivity

The Fire Triangle

Let's discuss what a fire must have in order to burn. There are three required elements needed to produce a fire. These three elements are commonly referred to as the fire triangle.

1. Oxygen - Enough oxygen must be present to sustain combustion.

2. Fuel - Some sort of flammable fuel source must be present. The minimum temperature need to ignite fuel is called the Flash Point.

3. Heat - Enough heat must be produced to make the fuel source ignite.

The flash point is the temperature that is needed to make a flammable liquid vaporize and burn. In order for a solid object to burn, the temperature must be high enough that it will decompose into a gas which is called pyrolysis.

Explosives

Whether it is a bomb in a crowded place, a gas leak within a house that ignites or a clandestine drug lab explosion, the experts in an explosives unit risk their lives daily to keep others safe from the dangers of explosive devices. Like fire, explosives  are also the result of an oxidation reaction except the explosive reaction is much more rapid. An explosion, or detonation, is the sudden and violent release of mechanical, chemical or nuclear energy from a confined space which creates a shock wave that travels at supersonic speeds. The rate or speeds of these reactions are what classify explosives as either high or low explosives. High explosives are substances that react faster than the speed of sound in the substance which makes them have a characteristic and very loud detonation. High explosives include dynamite, C4 and TNT and are used in commercial and military blasting. Because of the extensive potential for damage, these types of explosives are very tightly regulated. 

Low explosives are more readily available and used more often than high explosives. Black powder, fireworks and smokeless powder are classified as low explosives. These explosives react much slower than high explosives and thus react at a speed slower than the speed of sound in the substance. Because of this slower reaction speed, a low explosive does not have a technical "detonation", but a "deflagration". Deflagration is the extremely rapid burning of a material. This is much much faster than normal combustion, but slower than detonation. Without travelling faster than the speed of light, a reaction is not considered a detonation!

Explosives discharge an enormous amount of energy at the time of detonation or deflagration. This energy is sent out in a shock wave which expels not only pieces of the container it was held in, but in the case of bombs, any shrapnel, nails or other materials that may have been in the container with the explosive material. If the explosion occurs next to a building, some of the building materials such as wood or glass may be fragmented and propelled with the force of the blast. In addition to the risk of injury from flying objects during the blast, there is also significant risk of fire damage and injury to surrounding structures and victims. Incendiary devices are devices specifically created to start fires. An example of an incendiary device is a Molotov Cocktail.

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Analysis of Arson and Explosives Evidence

Analysis of Arson and Explosives Evidence

When evaluating the scene of an expected arson, investigators look for specific types of evidence and patterns. Once a fire has been extinguished, investigators can begin the process of examining the scene for signs of arson. The focus of the investigation should hinge on finding the point of origin to see if an accelerant or ignition device can be located. In most arson cases, a petroleum product is used for the accelerant and a match is used as the ignition device. Under normal circumstances fire tends to move up and out forming a V-shaped pattern from the point of origin. This V-shape occurs due to the way in which fire burns; out to the sides and vertically. The origin would be at the bottom point on the V. Not all fires are as straightforward, however! When the fire pattern is "ribbon shaped" from the point of origin, this is considered a red flag and arson is usually suspected. This pattern is typically produced when fire follows the path of an accelerant that has been poured or sprayed in the area. Structures such as stairwells also often block the path of the fire and force it in another direction, so an arson expert is often called in to verify any inconsistencies in these cases.

An arson expert can provide valuable insight in cases of arson; especially when there is more than one point of origin. An example of a fire scene with more than one point of origin is when trails of accelerant are left throughout the structure and then the accelerant is lit at multiple various points. Under extreme heat, such as when accelerants are used, investigators look for crackle patterns on the walls, ceilings and floors. A pattern that resembles alligator skin may also be found in these cases. The areas with the smallest "scales" of alligator skin patterning indicate that the fire was hottest in that area. Another indicator is that of the other building materials; glass melts and steel beams will buckle at 1500° F. These extreme temperatures are possibly indicative of the presence of an accelerant.

Once the point of origin has been located, any ash, soot, and porous materials is collected and stored in airtight containers. Often, new metal paint cans are used to contain the items of evidence for transport to the laboratory. It is important that the containers are airtight because the vapors of any accelerants present must be trapped for testing before they dissipate. If an accelerant was used, traces of flammable liquid residue may still be present on the collected material. This is because combustible liquids are almost never entirely consumed during a fire. It is also necessary to look for ignition devices such as matches, a sparking device, or parts of a "Molotov cocktail."

Once the evidence is in the lab, the technician will heat the can to release any accelerants that may be present. Once any accelerants are released, the technician will obtain a sample of the vapors released for testing by Gas Chromatography (GC) and Mass Spectrometry (MS). The analysis from the GC and MS will detail the chemical compounds found within the vapor. This method of testing is very accurate and sensitive and can detect amounts of accelerant as small as 1/1000 of a drop on a piece of evidence!

Collection and Analysis of Explosives

When collecting explosive evidence, the first priority of an explosives unit expert, or "Bomb Squad", is to find the explosive device or source and render it as safe as possible. Often this involves using remote control robotic devices to photograph the device, shoot a jet of water at the device to disrupt it, as well as safely detonating the device away from people and vulnerable structures. Often, xrays are used to examine suspicious packages or victims for traces of explosive device material such as metal shards in a container or shrapnel in a victim. Once the area where the explosion occurred has been declared safe, explosion site and surrounding area affected by the bomb should be systematically searched for trace evidence that can be collected for analysis in the laboratory. An Ion Mobility Spectrometer (IMS) is used to chemically detect explosive residues on objects in the explosion site. Any materials collected from an explosion site should be placed in sealed air-tight containers and labeled appropriately.

Laboratory testing will include microscopic analysis, high-performance liquid chromatography (HPLC), Gas Chromatography/Mass Spectrometry, xrays, and infrared spectrophotometry. If the explosion is suspected to be terrorist related, the FBI has a special lab that they use in order to examine the evidence. This lab is known as TEDAC, or Terrorist Explosive Device Analytical Center, and it is located at FBI Headquarters in Quantico, Virginia. Bomb signatures, or the "ingredients" used to make the bomb are kept in databases maintained by the FBI, ATF (Alcohol, Tobacco and Firearms) and the National Fire Incident Reporting System that can be used to analyze evidence from explosive sites. These databases are kept so that specific terrorist groups and serial bombers can be tracked. TEDAC analyzes IED evidence from all over the world!

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Properties of Firearms

Properties of Firearms

A firearm is a weapon, especially a pistol or rifle, capable of firing a projectile and using an explosive charge as a propellant. Essentially, this means that all firearms must have a charge material such as gunpowder, a projectile such as a bullet and a barrel to fire from. 

Firearm Anatomy

There are many different types of firearms; this module will focus on the differences between barrels that have been rifled, such as rifles and handguns, and shotguns which have not been rifled. Some of the most common types of guns found are rifles, pistols, revolvers and shotguns. A rifle is a shoulder firearm with a rifled barrel designed to fire one projectile, or bullet, at a time. A pistol is firearm designed to be held and fired with one hand. A revolver is a type of pistol having a revolving cylinder with several cartridge chambers that may be fired in succession. A shotgun is a firearm with a smoothbore, or smooth barrel without rifling, designed to fire small pellets called shot or rifled slugs. A different set of guns known as semi-automatic and fully automatic guns, are somewhat less common. A semiautomatic weapon fires one round with each individual trigger pull without the need to reload or reset the gun after each shot. A fully automatic weapon continuously fires rounds while the trigger is pressed and held. These weapons use a special type of ammunition held together so that they can be fired in rapid succession called a magazine. A magazine is the part of a repeating firearm which holds the cartridges or shells in position ready to be loaded one at a time into the chamber.

 Some of the major parts of a gun include the barrel, muzzle, breech, trigger, and hammer. The barrel is the metal tube of a firearm made from iron or steel, through which the bullet or shot charge passes when the firearm is fired, whereas the bore is the tunnel down the barrel of a firearm through which the projectiles travel. Gun barrels can be rifled or smooth; rifles and handguns have rifling whereas shotguns have smooth barrels. Rifling is the spiral grooves cut into the inside barrel surface to cause a bullet to spin, thereby stabilizing it. The cut-away portions of the rifling are called grooves and the uncut portions are called lands. The rifling causes the bullet to spin before it leaves the barrel, resulting in greater accuracy and range. Lands are the raised areas between two grooves in the rifling of a gun barrel that impart grooves on the bullet. Grooves are spiral cuts into the bore of a barrel that give the bullet its spin or rotation as it moves down the barrel. They look like depressions in the bore next to the raised lands. Caliber is the diameter of the bore of a rifle before the rifling grooves are cut. The rifling is used to increase both accuracy and range on a firearm. When Muskets had smooth bores, their range was only 50-75 yards with fairly poor accuracy. With rifling, Musket range increased to about 200-300 yards and accuracy also increased!

The muzzle is the forward end of a barrel. Historically, many of the earliest firearms were muzzle loaded which meant that the the ammunition was loaded from the front end of the barrel with a propellant such as gunpowder. While this method worked most of the time, it was time-consuming to load and did not allow for more than one shot per loading. It also was prone to misfires and led to more dangerous conditions.

The breech is the portion of the gun that contains the firing mechanism which is generally located at the back end of the barrel. Bullet cartridges are loaded into the breech end of the gun for firing. A cartridge is a case, usually made of brass or copper, containing the power charge, the primer and the bullet. It is also known as a shell casing. The firing pin is the part of the breech mechanism which strikes the primer of the cartridge. When a firing pin strikes the primer of the cartridge, it makes a unique marking on the back of the cartridge that can be examined to verify matching bullet cartridges. Interestingly, a method was developed in which the firing pins of modern firearms could be engraved by laser with microscopic markings or serial numbers so that they could be identified in Forensic investigations. While the method is still being refined, it could someday become commonplace to have each cartridge discharged with a unique serial number that could be used to link to a specific gun. The firing pin in a firearm is struck by a structure known as the hammer. The hammer is the part of a gun that hits the primer or firing pin, or explodes the percussion cap, and causes the gun to fire. The hammer is activated by pressing the trigger. The trigger of a gun is defined as the lever that activates the firing mechanism of a gun.

The "Sawed-off Shotgun" is a shotgun in which the barrel has literally been sawn off to 18 inches or less. This is done for one or more of the following reasons: to make the weapon easier to conceal, to make it easier to shoot one-handed and to force the shot to spread in a wider pattern when it exits the gun. While they these types of shortened shotguns are legal with special permits, they are of interest in Forensic investigations because of they are often used to commit crimes.

How a Firearm Works

Initially a cartridge enters the chamber and the firing pin is held back due to a spring. When the trigger is pulled, the spring releases sending the firing pin forward. The firing pin then hits the base of the cartridge, igniting the primer powder which sparks through the flash hole to the gunpowder. The gunpowder is the main propellant supply. The pressure of the exploding propellant pushes the bullet from the casing into the barrel. The bullet follows the lands and grooves produced by rifling and spirals out of the barrel. The casing is then ejected and a new bullet can enter the chamber.

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Analysis of Firearms and Ballistics Evidence

Analysis of Firearms and Ballistics Evidence

Firearm Identification

Forensic firearms identification is mainly concerned with determining whether a bullet or cartridge was fired by a particular weapon. To do this, a firearms expert will first evaluate the caliber of any bullets or casings found at the crime scene as well as the suspected weapon if it is in custody. Next, the casings will be inspected using a comparison microscope for any unique markings caused by the specific firing mechanisms within the gun and the way in which the gun was manufactured. Gun barrels are made by hollowing out a solid bar of steel with a drill. This process results in microscopic drill marks that are random and irregular. No two rifled barrels, even those manufactured in succession, will have identical rifling impressions. The rifling impressions contain microscopic striation  markings, or scratches, on the surface of the casing of the bullet. These subtle differences in the markings allow investigators to compare a bullet from a crime scene to a bullet that has been test fired from the weapon they suspect was used in the crime. Because these microscopic scratches are unique to a particular weapon, they are usually considered individual evidence. If the unique irregularities of the striations match, a link has been made to that particular weapon, and subsequently a possible suspect.

Casings can also be evaluated based on the impressions left by firing pins. While the markings left by firing pins are considered class evidence, matching firing pin marks can add support to any rifling impressions found.

Usually recovered bullets only yield a small area with intact markings. Grit, rust, and mutilation upon impact make it rare to find a perfect match all the way around a bullet. When collecting lodged bullets, investigators must be very careful to prevent additional scratches or mutilation.

In order to test a suspected weapon, investigators obtain bullets matching the same type and caliber from the crime scene and load them into the suspected weapon. The weapon is then test-fired in a special water chamber. The test-fired bullets are collected and evaluated for characteristic markings. Comparisons can then be made between the evidence bullets and the test-fired bullets to see if there are any matches.

In addition to bullet evidence, there may be other types of evidence found on a gun such as blood, fingerprints, or other trace evidence. Investigators must carefully examine the firearm for these other types of evidence which may be even more telling that the striations on a bullet! For example, finding the victim's blood (confirmed by DNA analysis) on a gun in possession of the suspect would be individual evidence.

Gunshot Residue

The firing of a weapon not only propels residues of gunpowder and other components toward the target, but also back toward the shooter resulting in trace deposits on the firing hand, nearby surfaces, and clothing. Thus, gunshot residue can provide very important evidence. Gunshot residue consists of burned and unburned gunpowder, vaporized, and particulate lead, and primer residues of lead, barium, and antimony. In an investigation, it is often necessary to determine the distance between the firearm and the target, as well as the distribution of powder patterns or the spread of shot pattern. Investigators make careful measurements and documentation of all visible gunshot residues to determine this information. The Griess Test is a chemical test that is used to develop patterns of gunshot residues around bullet holes on clothing or other fabric. It works by detecting nitrites in gunshot residue. Photo paper is placed over a bullet hole and heated with an iron containing a weak vinegar solution. The vinegar solution is applied to the area of the bullet hole using the steam feature on the iron. The vinegar reacts with the photo paper to make the nitrites visible as an orange color pattern on the photo paper. Because this method does not destroy the item of evidence, it is usually conducted first followed by other subsequent testing. A Sodium Rhodizonate test may be conducted to detect any lead residues in the areas of suspected gunshot residue. It is conducted by spraying the area of suspected gunshot residue with a solution of Sodium Rhodizonate. If lead residues are present, the sprayed area will turn a bright pink color on the item. To confirm the presence of lead, the bright pink areas are then covered with diluted hydrochloric acid. If the area turns blue in the presence of hydrochloric acid, the presence of lead is confirmed.

Once all testing is completed to locate and confirm gunshot residue, the investigators will then test the weapon with the same type of bullets at various distances to come up with a minimum and maximum distance required to reproduce the same general gunshot residue pattern. Shots are fired at various distances into targets holding white cotton and denim fabric. These cloth targets are inspected for gunshot residue patterns consistent with the evidence collected and can then be used as an approximation for the distance in which the gun was fired from the victim or object.

Ballistics

A very important part of firearms evidence investigations is ballistics. Ballistics is the study of what happens to moving projectiles in the barrel and in flight; their trajectory, force, impact, and penetration. The trajectory, or the path a bullet travels from muzzle to impact, of a projectile can be determined if two reference points for the projectile can be found. Reference points can be bullet holes in objects or victims such as an entry point and exit point on a victim. A bullet does not travel in a straight line over a long range, but rather in an arc; this is due to the force of gravity on the bullet. Investigators can use lasers to trace a straight-line path between the reference points to help determine the position of the shooter based on the idea that the shooter would have discharged the firearm somewhere along that line.

Collection and Preservation of Firearm Evidence

The most important consideration for collection of firearm evidence is SAFETY!

The best way to pick up a weapon is with 2 fingers on the trigger guard or the checkered portion of the grip which rarely yields prints. If a weapon is found in water, it should remain submerged in a suitable container in the same water to prevent rusting during transport.

When a semiautomatic weapon is used, the discharge pattern of ejected shells should be noted and documented carefully with photography. Carefully collect and pack clothing of gunshot victims to prevent loss of any gunshot residue. If the clothing is wet it should be allowed to air dry out of direct sunlight before packing. Each item should be bagged separately for transport to the lab.

Integrated Ballistic Identification System (IBIS)- is used to associate evidence in previously unsolved crimes. IBIS is a highly technical, computerized image analysis system that records striated images from bullets and cartridge cases and compares them to a national, and growing international, database of images.

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Toolmarks and Impressions

Tool Marks and Impressions

Tool Marks

A tool mark is any impression, scratch, or abrasion made when contact occurs between a tool and an object. Like firearms, tools also have irregular ridges and valleys as a result of manufacturing and/or use. Similarly, observing for striations on tools to compare to marks made at the crime scene can help link a particular tool to a crime scene and possibly a suspect. Some of the most common types of tool marks include abrasion marks, cutting marks, and indentation marks. Abrasion marks are made when one surface slides across another and essentially scratching the surface. Cutting marks are produced along the edge as a surface is cut. Indentation marks are made when a tool is pressed against a softer surface resulting in an impression of the harder tool in the softer material. This can be quite useful in identifying the tool!

Because continued use can change the striations on the tool and because the crime scene evidence could be altered, it is imperative that the tool never be put back in the crime scene mark to see if it matches. Instead a clay or putty casting should be used to mold a pattern of the weapon.

Other Impressions

Other impressions include tire treads, shoe prints, or fabric impressions. Tire treads and shoe prints can be preserved by casting them in plaster. Before casting the impressions should be photographed. Casts can then be collected and stored indefinitely. Fabric impressions can be made in a variety of ways. In fact, fabric impressions can be left behind as a result of a hit and run by the victims' clothing onto the surface of the accident vehicle. The primary consideration in collecting impressions is to preserve the impression or its reproduction for later examination in the crime lab.

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