What About the Crime Post-Blast?

by Andrea Campbell

The government defines a bombing as an incident in which an explosive or incendiary device has actually functioned. There are attempted bombings, of course, and premature explosions. The center of a bombing is referred to as the “seat.” (That's a historic photo of the Los Angeles Times bombing—October 1, 1910—at left.)

I started reading up on what it takes to work a post-blast crime scene, and though I found a lot of good information, I know it didn’t begin to scrape the surface. I guess that’s the wrong metaphor to use, because after a bomb goes off, there isn’t much surface left. Can you imagine going to the scene? There are tons of material called gross physical evidence. Most of the time you don’t know what you’re looking at, even though nearly everything is potential evidence. The debris is shredded, cut, spun, blended, burned, ruffled and any other adjective describing destruction like nothing you’ve ever seen.

Classifying and Typing

A bombing is deliberate or accidental. There are instances where these can be confused. Story has it that a man was charged with causing an explosion; he was in the business of running a grain silo. Anyone who knew anything about volatility should have known that grain dust explosions are common in silos. But back to business. There are three types of explosions: mechanical, nuclear and chemical. An example of a mechanical explosion might be a vessel -- such as a boiler or a propane tank -- under great pressure that gives at its weakest spot, dispersing its contents. Nuclear explosions are created by fission or fusion of unstable atoms, and chemical explosions are the result of either low or high explosives triggered by shock or heat.

A low explosive is typically heat-sensitive material that burns rapidly, about 1,000 feet per second. High explosives, on the other hand, are considered detonations — decomposition of molecules that are forced through a shock wave at about 10,000 feet per second. The difference between the two is significant in helping to understand what has happened; low explosives are more likely to cause a fire than the shorter impulse time associated with high explosives.

Explosives Differ in Other Ways Too

The chemicals needed for low explosions are generally substances such as black powder, pyrotechnic powders, and other fuels. High explosives include materials like dynamite, TNT, and RDX components such as C-4, those usually used by the military and for commercial applications such as imploding old buildings.

Low explosives need a housing or container, because burning produces the gases needed for expansion. High explosives give off their energy as the result of a detonation or shock wave and can be out in the open.

Bombing Materials

Bombs in criminal investigations are often called IEDs, short for improvised explosive devices. The explosive material can be commercially created or homemade. To initiate an explosion the material needs an igniter, which can be as simple as a burning fuse or as sophisticated as a complex electronic device. (The Oklahoma City bombing is pictured at right.)

The activator of the fusing system is put into three basic categories: 1.) time-activated, 2.) victim-activated, and 3.) command-activated. The first uses a time delay, the second is similar to a booby trap, and the third is set off manually, often using a remote-control device. According to GlobalSecurity.org, a person-borne suicide bomb usually employs a high-explosive/fragmentary effect and a command-detonation firing system -- a switch or button the suicide-bomber pushes to set off the blast.

Dr. Kirk Yeager, an explosives forensic scientist at the Federal Bureau of Investigation explainsit quite simply: A bomb consists of an oxidizer and a fuel. An explosion requires oxygen, provided by the oxidizer, and a fuel source, which can be as basic as sugar. A common oxidizer is ammonium nitrate, a primary ingredient of fertilizer. While bombs made with sugar and ammonium nitrate may be less potent than more advanced fuel sources, like TNT, the ingredients are easy to buy and legal to own. It’s interesting to note that the rapid oxidation in a bombing changes the colors of materials such as pipe or steel. (The 2004 Madrid train bombing is pictured above left.)

Working the Scene

A team will have a leader, a bomb technician, a photographer and sketch artist, and an evidence custodian. Depending on the size of the blast area, the team may be augmented by additional investigators.

The borders of a post-blast scene can be tremendous in size. Paul R. Laska, a retired crime scene investigator, says the lead investigator generally looks for the furthest item that can be identified as having originated at the point of the blast. A radius is established based on the distance of that item plus half again.

Search Methodology

Generally, the physical search and evidence collection are organized like a field search, with participants walking an approximate arm’s length (or wing span) apart in a grid, strip or even in a spiral pattern. Most common is the strip-style search, broken down into sectors.

Investigators don’t touch or collect the materials, but mark them out; the object is photographed where it lies. Everything is sketched, and an evidence custodian wearing gloves collects the items and often puts them into clean, unused paint cans, nylon bags, glass vials or sturdy cardboard boxes. The idea is to preserve DNA, fingerprints and explosive residue.

Searches are thought of as being three-dimensional because exploding bits can be lodged into walls, thrown onto roofs — don't forget people tossed into trees — and items driven into the ground. Yeager recalls a car whose pieces landed as high as 75 feet at a U.S. Embassy bombing.


Often investigators will suit up in protective gear including shoe covers to avoid blood-borne pathogens. The scene is monitored for hazardous materials as well.

Collecting and Preserving

Oftentimes the debris will be trucked to another area for examination. It’s taken to a secure facility where pieces can be sifted, examined for potential evidence, segregated, identified and put back together. Dr. Yeager said that he has spent a lot of his time going between different hardware stores, comparing their inventories to bomb components.

General Facts

• The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) has been collecting, storing and analyzing records on explosives and arson incidents since 1976.
• The FBI works on bombings related to terrorism.
• 1997 Omnibus Consolidated Appropriations Act, 18 U.S.C. 846(b), established a national repository for incidents involving arson and the criminal misuse of explosives.
• The U.S. Bomb Data Center (USBDC) is the sole repository and contains information on more than 180,000 arson and explosives incidents investigated by ATF and other federal, state and local law enforcement and fire investigation agencies.
  According to the US Bomb Data Fact Sheet, which provides overall statistics and information such as event locations, bomb types used, and regional maps, contains the figures listed below. Explosives Incidents in the United States: In 2007: 2,772 explosives incidents, 60 people injured, 15 killed, and 633 referred for prosecution. In 2006: 3,445 explosives incidents, 135 people injured, 14 killed, and 745 referred for prosecution.
  
  

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