Investigators in the Boston Marathon bombings have at least one advantage: a rich stream of possible evidence drawn not only from surveillance cameras but from personal photos and video footage, as well as cellphone records.
Authorities Tuesday said that they planned to sift through footage from surveillance cameras near Boston's Copley Square and that police had been assigned to review surveillance tapes from nearby businesses. The police commissioner, Ed Davis, vowed officials would go "through every frame of every video" taken during Monday's Boston Marathon.
"This is probably one of the most photographed areas in the country yesterday," Davis told reporters.
Investigators said they had already received a huge volume of tips, but appealed to the public to share any amateur photos or videos, as they might provide clues.
A federal law enforcement official said the FBI is also checking cellphone activity on towers near the scene of the blasts. Experts say that data could be particularly useful if the explosives were detonated by phone, especially since they know the exact time each blast occurred. If the tower data yield suspect numbers, investigators can obtain from the phone company subscriber information giving name, address and billing information. If the phone was a disposable model, it's possible to learn where the phone was purchased.
Surveillance cameras near the point of sale could provide further clues, some former federal prosecutors said.
The forensic investigation of the Boston Marathon bombing is developing a preliminary picture of how it was done, though not yet who may have done it.
Almost immediately, the two billowing clouds of white smoke visible in videos were a tipoff to bomb specialists about the type of explosives used. Within 24 hours, enough bomb fragments were retrieved by investigators to suggest how the improvised explosive devices were constructed.
The white smoke indicated that the bomber used so-called smokeless or black-powder explosives rather than a military-style high explosive such as C-4, which produces a distinctive black smoke, according to Fred Burton, former deputy chief of counterterrorism for the State Department's Diplomatic Security Service, who investigated the 1993 World Trade Center bombing.
"The real way to know what explosive was used is to analyze the post-blast debris for traces of explosive," Michael Sigman, assistant director for physical evidence at the National Center for Forensic Science at the University of Central Florida in Orlando, said in a phone interview. "There will be traces there."
Investigators can identify the chemical composition of the explosives quickly, even before residue samples are sent to the FBI's laboratory in Quantico, Va., he said. The analysis may provide some indications about its source, though these may be limited, particularly if it's a commercial material, he said.
Authorities will compare the bomb materials to a database of those used in other crimes and overseas in places such as Iraq and Afghanistan, as well as Internet postings and jihadist magazines that suggest how to create explosive devices, said Timothy Murphy, an FBI agent for 23 years who served as deputy director of the bureau.