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What Investigators Look for as They Comb Evidence in Boston Bombing

Investigators study the scene on Boylston Street at the site of the second bomb explosion. Photo by John Tlumacki/The Boston Globe via Getty Images.

A day after two blasts rocked the Boston Marathon finish line, investigators are scrambling to unearth clues on the devices and who planted them.

Some early details began emerging on Tuesday. The bombs seemed to have been fashioned out of kitchen pressure cookers, the Associated Press reported on Tuesday, citing investigators. They were likely packed with nails and other shrapnel, some of which has been found embedded in the bodies of the injured. Evidence of kitchen timers were also found.

Some reports said the bombs could have been hidden in a trash can, turning the trash cans themselves into weapons during the explosion. More recent reports suggested they were packed in backpacks or duffel bags and left on the street near the finish line.

“I would speculate that it’s some sort of a timed device,” said Frank Doyle, a former FBI intelligence analyst who was involved in the investigations of the Oklahoma City and New York World Trade Center bombings. “I would have even gone as far to say that both of these devices attempted to be set off at the same time.”

All of these elements will help investigators build what’s known as a “signature” of the perpetrator. That will help them hone in on who built this, where they were trained and where they got the materials, said A.J. Clark, a former military intelligence analyst, who says the device appeared to have been designed to create maximum damage to a large crowd.

In the immediate aftermath, investigators will search for remnants of the bomb — duct tape, for example, or any signs of explosive material. They’ll study indentations in the ground — which will give clues to how closely it was positioned to the road, buildings, a park bench or a trash can.

They’ll study videos and still photos captured by witnesses. These videos, Clark said, can prove “tremendously helpful.” Already, the white smoke in the videos indicates that the bomb was a homemade device, not military ammunition, dynamite or a C4 explosive, he said. Military explosives of that sort would produce darker smoke.

“It’s the chemical makeup,” Clark said. “You’ll find that stronger explosives have a denser, more explosive power, and they’re of a darker nature. Homemade chemicals give off a lighter smoke when detonated.”

Then they’ll start digging deeper into what’s known as biometric forensics. That involves looking for DNA, for example, or fingerprints — a thumbprint on a sticky part of duct tape, anything available to analyze the type of explosive material used.

“This isn’t anything too eloquent,” Clark said. ” Putting something inside a pressure cooker doesn’t add that much complexity to it.” Bombs designed using pressure cookers, he added, are commonly found in India, Pakistan and Afghanistan.

But Doyle disagrees with those calling it a “crude” bomb.

“It wasn’t crude because it functioned, it worked, it killed people, it obtained a goal,” he said. “It was complex enough to kill people, to do what bad people wanted to do.”

Eventually, all evidence will be taken to an FBI laboratory, where it will be closely analyzed, Doyle said. They’ll comb through swabbings and residues and they’ll methodically lay out components, comparing the two explosives and comparing elements thought to have come from the containers with material outside. All of information will be entered into a database to be used in future incidents. Meanwhile, investigators will likely pore through details on other bombings in search of useful patterns.

The 1996 explosion during the summer Olympics in Atlanta’s Centennial Olympic Park may provide the best model for Monday’s event, said Michael Greenberger, director of the University of Maryland’s Center for Health and Homeland Security.

“It’s a similar kind of episode,” he said. “A bomb that randomly went off with crowds of people around.” But, he added, with cell phone videos and other technology, forensics are better now.

In the case of the Atlanta attack, serial bomber Eric Rudolph wasn’t arrested until nine years later.

Greenberger said he’s optimistic that they’ll find the perpetrator of the Boston marathon bombings eventually, but possibly not until evidence is painstakingly pored through and analyzed.

“I have the unfortunate suspicion that this won’t be solved quickly,” he said.


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