Photo by David L. Ryan/The Boston Globe via Getty Images

Two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013.

(Photo by David L. Ryan/The Boston Globe via Getty Images)

Even as the police and doctors treat the wounded, the forensic investigation into the explosions at the Boston Marathon will begin.

"The forensics start as soon as people realize there's been an explosion," says Tom Thurman, of Eastern Kentucky University.

Thurman knows a lot about bomb investigations. Before his retirement from the FBI in 1998, Thurman was the chief of the FBI Bomb Data Center; he also worked Pan American Flight 103 over Lockerbie, Scotland; the bombing deaths of a federal judge in Alabama and an attorney in Georgia, both in 1989; and the 1993 bombing of the World Trade Center in New York.

The first thing to do is to determine if the explosions were intentional. "What's there that could spark an accidental explosion?" Thurman asks. If no likely sources for an accidental detonation are found—like a buildup of flammable vapors—the investigators start looking at other evidence.

The Boston Globe is reporting via Twitter that a third device was found, unexploded, that police are detonating intentionally. So the fact that the scene in Boston is a mass homicide is now obvious.

Video will be crucial to determining what happened in Boston, much more than the laboratory analysis, Thurmon says. "They will be looking at how the bomb got there: who deposited it and when."

Even the video of the blast can help identify what kind of bomb it is—or in the case of Boston, confirm that the bombs that detonated were the same that went off. "Generally, white smoke means a commercial explosion or improvised device," he says. A common chemical used in these bombs, in the United States and abroad, is acetone peroxide (TATP). It comes in a white powder and blooms in a white cloud when it explodes. In Boston, the initial images seem to show white smoke blossoming at the moment of explosion.

Industrial and military explosives emit black smoke, Thurman says.

If the video proves inconclusive, there are other ways to figure out what happened. One main question is whether it was a suicide bombing or a remote-control device. "There is a very discernible difference between the injuries of a suicide carrier than of other victims," Thurman says.

Re-creating the injuries will help determine the direction of the shrapnel, and help locate the epicenters of the devices—and that means detailing injuries to living victims and examining the deceased, he says. Human bodies that are hit by shrapnel have evidence in their bodies. All that information should be chronicled by investigators, on the scene, in the hospital and morgue. Residue of explosions needs to be collected and sent to the lab—devices can be tested in the field for their composition, but residue cannot, Thurman says.

He cautions not to be too hasty in assessing blame. "Let the evidence direct us," he says. "We need to have an open mind. This could have been anybody."

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