WASHINGTON —The bomb investigators swarming Boston are combining high-tech tools with old-fashioned shoe leather as they piece together what blew up and why.

A special federal bomb squad has mobilized, joining state and local counterparts in a search for everything from the shrapnel that slashed victims to the residue left behind after dual blasts Monday killed three and injured more than 170 near the finish line of the Boston Marathon. It’s painstaking work, combining chemistry, computer databases and sheer doggedness.

Richard DesLauriers, special agent-in-charge of the bureau’s Boston division, appealed to the public late Tuesday to produce any information about anyone seen carrying a dark heavy bag at the scene of the bombing or who threatened an attack on the Boston Marathon. He also sought information about explosions heard in remote areas where the bomber might have conducted tests.

“We are doing this methodically, carefully, yet with a sense of urgency,” he said. “Someone knows who did this. Cooperation from the community will play a crucial role in this investigation.”

Gene Marquez, acting special agent-in-charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives’ Boston field office, said that the bombing scene would take “several days to process.”

Some clues come from what doctors pull from the victims’ bodies. Doctors on Tuesday reported that they have been extracting objects that appeared to be pellets and nails from the legs and torsos of victims, a possible sign that the two bombs that exploded Monday had been destructively packed.

“One of the sickest things for me was just to see nails sticking out of a little girl’s body,” Dr. David Mooney, director of the trauma center at Boston Children’s Hospital, told reporters at a morning briefing.

A former senior U.S. government official who was briefed on the investigation but declined to be identified because of the sensitivity of the information, confirmed for McClatchy Newspapers that the bomb was put into a pressure cooker, a tactic that counterterrorism agencies have found in the past in jihadist plans and “recipes.”

No one has claimed credit for the bombings.

Following an Oval Office meeting Tuesday with President Barack Obama and his top national security advisers, Homeland Security Secretary Janet Napolitano said there was no evidence that the bombings were part of a broader scheme.

Among the lines of inquiry are that the attack was the work of domestic terrorists or a lone wolf, said a person familiar with the investigation, who also asked for anonymity because of the sensitive nature of the information.

The Department of Homeland Security issued a bulletin around 2005 warning of the risks of pressure cookers in explosive devices after recovering jihadist literature describing the tactic.

The Boston Globe reported midafternoon Tuesday that investigators had found a circuit board believed to have been used in the detonation of the bombs. Citing an individual briefed on the investigation, The Associated Press further reported Tuesday afternoon that investigators believe the two bombs were hidden inside black duffel bags.

“We have only two devices that we are aware of and both were the devices involved in the damage and explosive incidents,” Marquez said, responding to reports Monday that additional devices had been found.

An often politically embattled part of the Justice Department and periodically targeted for elimination by conservatives, the Bureau of Alcohol, Tobacco, Firearms and Explosives, still known as the ATF, is playing a crucial but subordinate role in the investigation. 

The FBI, an occasional bureaucratic rival that’s also part of the Justice Department, is the lead agency.

DesLauriers said Tuesday that the bureau and the multi-agency Joint Terrorism Task Force with which it’s working have received “voluminous tips” since the explosion. Boston Police Commissioner Edward Davis III said that the blast area is “the most complex crime scene in the history of our department.”

About 30 ATF investigators are on the scene, including members of what’s called the National Response Team, called up to aid the Boston Police Department’s bomb squad. 

The national team includes special agents, forensic chemists, canine handlers and bomb technicians, known as explosives enforcement officers. They deploy from a fleet of white-and-blue trucks loaded with specialized equipment and have previously been called up for manmade catastrophes, including the April 1995 Oklahoma City bombing.

The turf and personality clashes that can sometimes shadow other joint investigations are eased, veteran investigators say, by the fact that every bomb technician from every civilian agency has graduated from the same Hazardous Devices School, run jointly by the Army and the FBI at Redstone Arsenal in Huntsville, Ala.

“When we go on the scene, we’re working side by side with people who went to the same school,” said Barney T. Villa, an independent security consultant and former bomb squad member in the Los Angeles County Sheriff’s Department. “We all have mutual respect for each other.”

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