| Friday, June 29, 2007 marked another day in London's | | | | High explosives can be divided into two categories all |
| history in which a terror scare rocked the people of | | | | dependent upon their sensitivity to heat, friction, or |
| this city. Fortunately, nobody was hurt unlike the last | | | | mechanical shock. |
| bombing that struck a London public transport system | | | | - Initiating explosives are highly sensitive to these |
| nearly two years ago next week. Two car bombs | | | | effects. Because of their instability, home-made bombs |
| were discovered and quickly defused by a Scotland | | | | rarely use initiating explosives. These explosives are |
| Yard bomb squad. Now the hunt goes on to find the | | | | usually found in primers and blasting caps, where they |
| perpetrators who could have caused such mass | | | | initiate other more stable noninitiating explosive |
| destruction of innocent lives. | | | | reactants. Mercury fulminate and lead azide are widely |
| If these car bombs did explode, how do forensic | | | | used in this manner. |
| criminal investigators go about evaluating this particular | | | | - Noninitiating explosives are more stable and |
| crime scene? In this article, we will attempt to explain | | | | commonly used in military and commercial applications. |
| how the investigators evaluate explosive situations, | | | | Examples of these explosives include dynamite, |
| how they define explosives, and how they go about | | | | trinitrotoluene (TNT), pentaerythritrol tetranitrate (RDX), |
| investigating a bomb scene. | | | | and cyclotrimethylenetrinitramine (PETN). ANFO, an |
| Explosions and fires are similar chemical reactions that | | | | easily made explosive material, is a mixture of |
| take place since both result from the consumption of | | | | ammonium nitrate and fuel oil. |
| fuel and oxygen. The only difference between the two | | | | Ammonium nitrate is an oxidant loaded with oxygen |
| is that an explosion reaction occurs faster than a fire | | | | and can be found in fertilizers. Bombs made from this |
| reaction. Explosion reactions use up their fuel such as | | | | material were used in the Oklahoma City and 1993 |
| gasoline or gunpowder almost instantaneously partly | | | | World Trade Center bombings. |
| because the materials are confined to a small space. | | | | Investigating a Bomb Scene |
| Fire reactions, on the other hand, consume their fuel | | | | Looking through the scene of an explosion requires the |
| such as wood, trees, or paper more slowly than | | | | same attention to detail as does a search of a fire |
| explosion reactions. If you ignited these materials in an | | | | scene. Searching for remnants of an explosive device |
| open space, the materials simply burn. In contrast, if | | | | such as the igniter and timer may be important in |
| you tightly pack these same materials into a container, | | | | determining the type of explosive used and the |
| it explodes when you light it up. | | | | persons responsible for this crime. Furthermore, |
| Explosions pose problems for investigators. The | | | | forensic criminal investigators focus their searches on |
| explosive device and any surrounding structures are | | | | gathering debris to test for unexploded residue, which |
| heavily damaged if not entirely obliterated. Unless a | | | | is almost always present. |
| secondary fire occurs, investigators can usually | | | | Microscopic examination of the debris may unveil black |
| ascertain the point of origin with no problem. Finding | | | | powder, gun powder, or both of which are easily |
| fragments of the device, timers, or igniters is another | | | | recognizable by the color and conformation of their |
| story. | | | | particles. After conducting a microscopic inspection of |
| Explosives are categorized as either high or low | | | | the debris, the lab technician rinses the debris with a |
| according to the speed of their resulting shock wave. | | | | solvent, usually acetone, and then analyzes the |
| Low explosives usually move at rates of up to 1,000 m | | | | resulting solution, using various scientific laboratory |
| s, and high explosives may reach speeds up to 8,500 | | | | techniques such as thin-layer (TLC) or gas |
| m/s. | | | | chromatography (GC) and mass and infrared |
| Black powder and smokeless gunpowder are the | | | | spectroscopy. Determining the identity of the explosive |
| most readily available and commonly used low | | | | is made through a combination of these techniques. |
| explosives. A mixture of table sugar and potassium | | | | After determining the nature of the explosive used, |
| chlorate makes another easy explosive. Bombers do | | | | criminal investigators then target their investigation on |
| not need to be complex. | | | | the seller and buyer of that explosive. |