BOSTON - The investigators swarming Boston are combining modern tools with old-fashioned shoe leather as they piece together what blew up and who might be responsible.
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 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 bag at the scene of the bombing.
"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."
Investigators also have been blessed with at least one advantage: a rich stream of possible evidence drawn not only from surveillance cameras, but also from personal photos and video footage, often from cellphones.
Authorities said that they plan to sift through footage from near Copley Square and that police have been assigned to review tapes from businesses. Boston Police Commissioner Ed Davis vowed that officials would go "through every frame of every video."
"This is probably one of the most photographed areas in the country (Monday)," he said.
Officials said they have received a huge volume of tips but appealed to the public to keep sharing - videos, smartphone photos, things they saw, anything that might provide clues.
Investigators said they do not have any suspects.
"I would encourage you to bring forward anything," said Col. Timothy Alben, superintendent of the Massachusetts State Police.
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. Surgeons revealed that they have been extracting pellets and nails from the legs and torsos of victims, a sign that the two bombs were destructively packed, as expected.
"One of the sickest things for me was just to see nails sticking out of a little girl's body," said Dr. David Mooney, director of the trauma center at Boston Children's Hospital.
Several government officials confirmed that at least one bomb was placed in a pressure cooker, a tactic that counterterrorism agencies have found in the past in jihadist plans and "recipes."
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 Tuesday that investigators had found a circuit board believed to have been used in the detonation, and that the two bombs were 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.
Davis said 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 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.
After the investigators are done picking through Copley Square, all of the information they collect can be compared to the 185,000-plus arson and explosive incident reports stored in the U.S. Bomb Data Center, the largest database of its kind.
"One (goal) is a forensic examination of the debris at the scene in an attempt to collect and ultimately reconstruct the design of the devices themselves," explained Brian Jenkins, a longtime counterterrorism expert.
The Washington Post contributed.