Forensic Science Info on Bombs - Evaluating a Bomb Scene

By Fabiola Castillo 

Friday, June 29, 2007 marked another day in London's history in which a terror scare rocked the people of this city. Fortunately, nobody was hurt unlike the last bombing that struck a London public transport system nearly two years ago next week. Two car bombs were discovered and quickly defused by a Scotland Yard bomb squad. Now the hunt goes on to find the perpetrators who could have caused such mass destruction of innocent lives.

If these car bombs did explode, how do forensic criminal investigators go about evaluating this particular crime scene? In this article, we will attempt to explain how the investigators evaluate explosive situations, how they define explosives, and how they go about investigating a bomb scene.

Explosions and fires are similar chemical reactions that take place since both result from the consumption of fuel and oxygen. The only difference between the two is that an explosion reaction occurs faster than a fire reaction.

Explosion reactions use up their fuel such as gasoline or gunpowder almost instantaneously partly because the materials are confined to a small space. Fire reactions, on the other hand, consume their fuel such as wood, trees, or paper more slowly than explosion reactions. If you ignited these materials in an open space, the materials simply burn. In contrast, if you tightly pack these same materials into a container, it explodes when you light it up.

Explosions pose problems for investigators. The explosive device and any surrounding structures are heavily damaged if not entirely obliterated. Unless a secondary fire occurs, investigators can usually ascertain the point of origin with no problem. Finding fragments of the device, timers, or igniters is another story.

Explosives are categorized as either high or low according to the speed of their resulting shock wave. Low explosives usually move at rates of up to 1,000 m/s, and high explosives may reach speeds up to 8,500 m/s.

Black powder and smokeless gunpowder are the most readily available and commonly used low explosives. A mixture of table sugar and potassium chlorate makes another easy explosive. Bombers do not need to be complex.

High explosives can be divided into two categories all dependent upon their sensitivity to heat, friction, or mechanical shock.

• Initiating explosives are highly sensitive to these effects. Because of their instability, home-made bombs rarely use initiating explosives. These explosives are usually found in primers and blasting caps, where they initiate other more stable noninitiating explosive reactants. Mercury fulminate and lead azide are widely used in this manner.
• Noninitiating explosives are more stable and commonly used in military and commercial applications. Examples of these explosives include dynamite, trinitrotoluene (TNT), pentaerythritrol tetranitrate (RDX), and cyclotrimethylenetrinitramine (PETN). ANFO, an easily made explosive material, is a mixture of ammonium nitrate and fuel oil.

Ammonium nitrate is an oxidant loaded with oxygen and can be found in fertilizers. Bombs made from this material were used in the Oklahoma City and 1993 World Trade Center bombings.

Investigating a Bomb Scene

Looking through the scene of an explosion requires the same attention to detail as does a search of a fire scene. Searching for remnants of an explosive device such as the igniter and timer may be important in determining the type of explosive used and the persons responsible for this crime. Furthermore, forensic criminal investigators focus their searches on gathering debris to test for unexploded residue, which is almost always present.

Microscopic examination of the debris may unveil black powder, gun powder, or both of which are easily recognizable by the color and conformation of their particles. After conducting a microscopic inspection of the debris, the lab technician rinses the debris with a solvent, usually acetone, and then analyzes the resulting solution, using various scientific laboratory techniques such as thin-layer (TLC) or gas chromatography (GC) and mass and infrared spectroscopy. Determining the identity of the explosive is made through a combination of these techniques.

After determining the nature of the explosive used, criminal investigators then target their investigation on the seller and buyer of that explosive.