The bureau's website offers instructions on how to submit footage of interest in the investigation, and the effort to gather it is so extensive that security officials at Logan Airport have been asking travelers leaving Boston whether they have data that could be useful.
Officials said they are organizing the video footage and digital images according to precisely where and when they were taken, relying on an array of clues to determine locations along the race route and the time that specific pictures were snapped. The fact that the bombing occurred at a marathon — an event that records runners' finish times in fractions of a second — has aided the effort. Investigators have used the numbers worn by runners to determine their finish times and then extrapolated backward to determine when they crossed key landmarks on Boylston Street, where the blasts occurred.
Pictures taken by cellphones also typically have time stamps and, in some cases, GPS location data.
"It's like a puzzle, but it's in color and it's four-dimensional because of the time element," said a U.S. intelligence official monitoring the investigation.
Investigators are reconstructing scenes of the explosions from as many angles as possible, seeking to narrow their search to the window during which the devices were placed.
One U.S. official compared the effort to the massive endeavor after Osama bin Laden's 2011 killing in Pakistan to sift quickly through the files and computer records recovered from his compound — except that in this case, the FBI, not the CIA, is in the lead role.
Often, investigators scrutinizing this kind of video evidence will use software that lets them add specific descriptions of a person they have observed in some of the footage and other data to attempt to find the individual in other videos from the scene, according to Grant Fredericks, a forensic video analyst who has taught classes for the FBI.