Bomb Damage, Forensic Assessment
█ JUDSON KNIGHT
Just as fires and explosions are closely related phenomena in physical and chemical terms, bomb-damage assessment is an aspect of forensic science closely related to arson investigation. In both cases, authorities analyze a crime scene for telltale signs of the nature of the materials that facilitated the conflagration. In the United States, the two agencies most concerned with bomb-damage assessment at the federal level are the Bureau of Alcohol, Tobacco, and Firearms (ATF), and the Explosives Unit of the Federal Bureau of Investigation (FBI).
Both fires and explosions involve a physical change in materials, such as the conversion of solid or liquid into gas, as well as the conversion of small quantities of matter into energy. Additionally, these processes involve a chemical change or reaction, that is, a rearrangement of atoms. Both processes must take place in the presence of oxygen, which is among the most reactive of the chemical elements, meaning that it is highly likely to bond with atoms of other elements.
During the process of oxidation, an element bonding with oxygen loses electrons, while the oxygen gains electrons, a process chemically known as reduction. The world is full of oxidation-reduction reactions, some of which include the rusting or corrosion of metals, the metabolism of food and other biological processes, and combustion. The last of these is commonly known as the process by which materials catch fire, and explosion is simply a fast form of combustion. In the combustion process, chemical bonds are broken quickly, releasing energy that is experienced in the form of heat. In the case of explosion, these bonds are broken even more quickly, producing even more heat and more kinetic energy, which propels objects outward from the center of the blast with greater impact.
Investigating a crime scene. The investigator of a scene where a bombing has taken place must be schooled both in basic physics and chemistry, but also forensic science, or the application of scientific techniques for the purpose of solving crimes. One of the first matters of interest to the investigator, obviously, is the nature of the explosive itself. At the low end of the spectrum are unsophisticated devices such as pipe bombs, which are usually little more than metal pipe containing black powder from shotgun shells.
Much more complex are explosives using TNT (trinitoluene) or nitroglycerin. The latter is found in dynamite, which combines sodium nitrate, nitroglycerin, and inert compounds. One notorious variety of explosive is ammonium nitrate, used in the 1993 World Trade Center bombing and the 1995 Oklahoma City bombing. Combined with fuel oil, it is known as ANFO, a foul-smelling— and lethal—sludge.
One difference between lower-level explosives and their more sophisticated cousins is the fact that the latter requires a detonator or blasting cap, a device to make it active. Investigators will, therefore, seek not only the telltale physical and chemical residue that will lead to a determination of the type of bomb used, but also for evidence of detonators and other components such as tapes, wires, timers, switches, and batteries.
Agencies and bombings. ATF agents investigating the first World Trade Center bombing, which killed six people, found a great deal of chemical evidence in the aftermath, ranging from the acrid, acidic smell of the air to specific types of molecular residue. There was also physical evidence that identified the perpetrators' van as the site where the blast originated: among the items noted were "feathering," or the fact of being stretched by the blast; "bluing," exposure to welding-torch-like heat; and "dimpling," whereby the metal close to the blast liquefied and shot out, colliding with nearby objects and leaving tiny craters on their surfaces.
In addition to the ATF, the FBI operates a laboratory to which other law-enforcement agencies submit materials for investigation. At the international level, bomb damage assessment may be performed by security services of various nations, or even by international teams, which may include civilians. Such was the case in the investigation of the scene in Bali, Indonesia, where Islamist terrorists detonated a bomb that killed several hundred people in October 2002.
█ FURTHER READING:
Bolz, Frank, et al. The Counterterrorism Handbook: Tactics, Procedures, and Techniques. Boca Raton, FL: CRC Press, 2002.
BBC News. Q&A: Bali Forensic Challenge. < http://news.bbc.co.uk/2/hi/asia-pacific/2327687.stm > (January 16,2003).
Bureau of Alcohol, Tobacco, and Firearms. Arson and Explosives Programs. < http://www.atf.treas.gov/explarson/index.htm > (January 16, 2003).
FBI Laboratory Explosives Unit. < http://www.fbi.gov/hq/lab/org/eu.htm > (January 16, 2003).