Washington — Investigators are convinced that the explosive devices that killed three people at the Boston Marathon were not in place before the race began because of sweeps by security teams. And since Monday they have focused on increasingly narrow slices of the five-hour window between when the marathon started and the blasts occurred, according to U.S. officials.
This methodical dissection of terabytes of video, still images and other evidence led the FBI to images of a potential suspect Wednesday. The apparent breakthrough illustrates how private surveillance equipment, in combination with the cellphone cameras used by ordinary citizens, has become an extraordinary resource that allows investigators to re-create the visual narrative of the streetscape surrounding a location in order to scrutinize the hours, minutes and seconds ticking down to a crime.
"There is absolutely going to be video of almost every single inch, for every single second of that day; it's just a matter of finding it," said Andrew Obuchowski, a former Massachusetts police officer who analyzes video evidence for Navigant Consulting, a private firm.
Video has proven crucial to a number of high-profile international investigations in recent years. In Dubai in 2010, the suspected assassins of a senior figure in the Palestinian group Hamas were captured on hallway video cameras in the hotel where the target, Mahmoud al-Mabhouh, was staying. In London in 2006, the city's ubiquitous closed-circuit video cameras were used to trace the movements of Alexander Litvinenko, a former Russian security agent, on the day he was poisoned with a radioactive isotope.
Along with the forensic examination of residue from the bombs, U.S. officials said, investigators are focusing heavily on the enormous volume of video footage and images taken by security cameras and spectators and turned over to the FBI.