Fires and explosions are closely related phenomena in physical and chemical terms. Appropriately, bomb-damage assessment is an aspect of forensic science closely related to arson investigation.
Explosions can be accidental, such as the rupture and ignition of a propane tank, yet they can also be the deliberate result of the detonation of a bomb. In assessing a crime scene, forensic scientists look for the telltale signs of the cause of the bomb damage.
Such forensic investigations can take place at the municipal and state level, if the involved agencies have the capability to conduct the investigations. Certainly, such capability exists at the national level. 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).
Explosions involve a physical change in materials. A solid or a liquid is converted into a gas or, given the tremendous force of some bombs, directly into energy. 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 force.
The investigator of a scene where a bombing has taken place must be schooled in basic physics and chemistry and in forensic science. An initial concern is to ascertain how the explosion occurred and what caused the explosion.
Some bombs are relatively unsophisticated. One example is a pipe bomb, which is typically little more than a metal pipe containing shotgun powder.
Much more complex are explosives using trinitrotoluene (TNT) or nitroglycerin. The latter is a component of dynamite, which combines sodium nitrate and inert compounds to generate the explosive punch. One notorious variety of an explosive is ammonium nitrate. This common ingredient found in fertilizer was used in the 1993 World Trade Center bombing and the 1995 Oklahoma City bombing. Combined with fuel oil, it produces a foul smelling and lethal ammonium nitrate and fuel oil (ANFO) 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
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 a lingering acrid and acidic aroma to the presence of specific types of molecular residues. There was also physical evidence that identified the perpetrators' van as the site from which the blast originated: among the items noted were "feathering," blast-related stretching; "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.
SEE ALSO Accelerant; Architecture and structural analysis; Bomb detection devices; Bomb (explosion) investigations; Explosives.
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