WASHINGTON — FBI bomb technicians poring over hundreds of scraps of metal, nails, wires, and other debris — some surgically removed from bomb victims’ flesh — were closing in Thursday on the design of the explosive devices used in the Boston Marathon attacks, according to officials and forensics experts.
In an effort to trace the source of the components, the Explosives Unit at the agency’s state-of-the-art crime laboratory in Quantico, Va., outside of Washington, was comparing the materials collected from sidewalks, rooftops, gutters, overhangs, and even the soles of victims’ shoes against lab reports generated from thousands of previous blasts around the world, looking for a “bomber signature.”
“I think that they should be coming out any day now with the results,” said Representative Michael McCaul, Republican of Texas and chairman of the House Homeland Security Committee.
The efforts parallel those in Boston, where investigators are sifting through thousands of images, including on videos, and eyewitness reports from the Monday attacks.
The methodical work at Quantico is seen as critical in helping investigators identify the perpetrators and secure a conviction in the worst terrorist attack on US soil since Sept. 11, 2001.
“Most bombing investigations like this are forensically driven,” said former supervisory special agent James T. Thurman, who served as chief of the Explosives Unit in the FBI Laboratory’s Bomb Data Center. “Whatever is found that ultimately goes to the laboratory is what drives the investigation and connects a possible subject with the components.”
McCaul, a former prosecutor who has been regularly briefed on the progress of the bombing probe, said he has been told by investigators that they are confident the bomb analysis will provide new information to propel the investigation forward.
“That will release a volume of information in terms of tracing where the gunpowder was purchased [for the bombs], where the ball bearings were manufactured, where the actual [cooking] pot, or device itself, with a serial number, was manufactured and bought,” he said. “From there they can look at possible search warrants.”
McCaul plans to visit Boston on Friday to huddle with local authorities.
An FBI spokeswoman in Quantico, Ann Todd, said agency officials could not be made available Thursday for interviews about the lab’s procedures, “in deference to the ongoing investigation being conducted by law enforcement authorities in Boston” and to avoid “unintended challenges for investigators during the early stages of this investigation.”
But those familiar with the inquiry and the FBI’s lab said the Explosives Unit is relying on specialized tools to conduct what amounts to an autopsy on the pair of so-called IEDs — improvised explosive devices — that were apparently detonated by battery-powered timers near the Copley Square finish line.
One is a database known as the Explosive Reference Tool, a searchable computer archive containing investigation reports from bombings dating back decades, along with so-called underground bomb-making handbooks and manufacturing data for key bomb-making components and explosives.
According to one former FBI explosives expert, the findings also will be compared with thousands of research reports from IED attacks overseas to determine if the Boston bomb makers could have received foreign training.
That part of the investigation will be handled by the Terrorist Explosive Device Analytical Center, a separate outfit at Quantico established in 2004 “to gather and share intelligence about these devices — helping to disarm and disrupt IEDs, link them to their makers, and most importantly, to prevent future attacks.’’
“Having the database from Iraq and Afghanistan and other bombing events around the world is critical,” said K. Jack Riley, vice president of the national security division at the government-funded Rand Corp., which has advised the Pentagon in its multibillion dollar effort to counter roadside bombs on the battlefield, the biggest killer of US troops.
The government also has a trove of data from domestic bombings. While most are relatively small and don’t make national headlines, hundreds of episodes involving so-called incendiary devices — such as pipe bombs — take place each year in the United States, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives.
A key early objective, Riley said, is to determine the nature of the explosive used — be it plastic explosive, TNT, or gunpowder.
“That information alone can move the investigation in critical directions quite significantly,” he said.
Then comes the identification of other material, such as nails, ball bearings, and other shrapnel, along with the timing and ignition devices, which can provide leads on location and time of purchase.
“Third, they are looking for anything that is the signature of the person who actually built the bomb,” he said. “You might get lucky.”
The bomb lab’s role began immediately after the blasts, when evidence response teams began determining how far away debris from the bombs were dispersed. They constructed a series of grids starting from the outer ring of the blast area and moving steadily toward the apparent location of the bombs to avoid contaminating or destroying any of the evidence by trampling through the site.
Potential evidence that was identified, bagged, and tagged — including by agents who spread out to hospitals to secure bomb material and residue removed from wounds — began arriving in the Quantico lab even before the bombing sites were fully swept for evidence.
“It is a two-way street,” said Thurman, who investigated the bombings of the US Marine Corps barracks in Lebanon in 1983, Pan Am Flight 103 in 1988, and World Trade Center in 1993. He explained that as the lab uncovers clues and materials it may send orders back to the bombing scene to look for specific things.
“They will continue that as long as it takes,” he added. “It can take less than a day. It can take a week. It can take two weeks.”
Thurman, who teaches at Eastern Kentucky University, also cautioned that it could take longer, citing the bombing of a judge in Alabama in 1988, when the forensics analysis took two years.
“This is not an overnight thing,” he said. “The issue at the end of the day is to find the guilty party who is responsible for constructing and setting off these devices,” he said. “You are ensuring that the right people and not the wrong people are being charged.”Bryan Bender can be reached at email@example.com. Follow him on Twitter @GlobeBender