(FBTA) Analysis of Arson and Explosives Evidence Lesson

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, x-rays 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, x-rays, 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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