Detectives scour video, photos for data

Photos of two people of interest were revealed during a press conference Thursday that included the key law enforcement officials investigating the Boston Marathon explosions that occurred on Monday.Barry Chin/ Globe Staff/Globe Staff

Law enforcement officers have spent countless hours since Monday poring through video footage and photographs, analyzing images of the final moments before two explosions rocked the finish of the Boston Marathon.

More than 100 Boston police detectives in the department’s headquarters in Roxbury have combed through the images, in addition to the work done by federal and state officers assigned to the case.

“Detectives are scouring video all the time,” said a law enforcement official with knowledge of the inves­tigation. “They are working in 12-hour shifts.”

The government also enlisted an assortment of other agencies to help scour the images, including the US Immigration and Customs Enforcement’s Homeland Security Investigations office in Boston, which normally tracks down child pornographers and victims, often through the painstaking review of video.

The fruits of their labor were revealed Thursday when the FBI released the first images of suspects in the deadly bombing.

In the hours and days after the bombing, Boston police visited stores in the area surrounding the blasts to retrieve extensive surveillance video taken before the explosions, which killed three people and injured more than 170 people. In addition, federal and local authorities have collected many more images from spectators who took pictures and video of their family, friends, and runners enjoying the race before the blasts ended the celebration.

The FBI declined to comment Thursday on how many images they have collected so far and would not discuss their efforts to gather and comb through the evidence. But they renewed calls for people to submit photos and video of the area near Monday’s explosions.

Detectives are also personally visiting people who submit images to ensure that nothing is forgotten or overlooked.

“Wherever the witness is, we’re going to them to guarantee that we get everything they have,” accord­ing to a second law enforcement official. The FBI asked people with tips or images to call 800-CALL-FBI or use a new web site, bostonmarathontips.fbi.gov.

Experts said that while tipsters may assume police only want images showing someone with a suspicious bag about the time of the blast, they are actually trying to cast a broader net for all video and ­images from the race, to make sure they capture any hidden clues about who might be involved in the bombing.

The enormous volume of images and video capturing the race is both a blessing and a challenge. But authorities do not have to look through each and every frame to begin identifying potential suspects, say terrorism and forensic video specialists. Instead, law enforcement authorities can zero in on the best video showing the street where the bombs were planted shortly before the explosions. The FBI said Thursday that the bombs were planted just minutes before the explosions.

“The FBI has done triage on the video, concentrating on those feeds that might offer the most information about who placed the bombs and when,” said Richard Clarke, who worked in the White House for years on terrorism and security issues and is an adjunct lecturer in public policy at Harvard University’s Kennedy School of Government.

The authorities also have access to ever-improving technology that makes it easier to sort through the material. For instance, software companies such as Burlington-based Avid Technology Inc. make programs widely used in law enforcement to help catalog photos and videos by time and locations. Some of the programs can even let investigators search for certain shapes or colors, making it faster to find any bags resembling those that carried the bombs.

“The technology is much more sophisticated today,” said Brian Michael Jenkins, a terrorism specialist for the Rand Corp., a nonprofit research firm.

But investigators probably started with a lower-tech approach: looking at the video of the locations where the bombs were left to see whether they captured any signs of who planted the bombs shortly before the explosions.

“It’s not a complicated process,” said Grant Fredericks, an analyst at Forensic Video Solutions of ­Spokane, Wash. “You start with the explosion . . . and you simply go backward in time. You don’t have to go back very far to find the package that was left.”

It could still ultimately take months to process and analyze all the video and photos investigators have gathered, everything from closed-circuit security cameras on rooftops to grainy snapshots from cheap cellphones.

But that is something investigators are likely to do in order to capture the full picture of what transpired that day, to identify any other potential suspects, and ultimately to prepare a criminal case against them.

“This isn’t the end of the investigation,” he said. “It’s the very beginning.”

This is not the first time investigators have looked through daunting amounts of video and photos to find suspects. After riots broke out in Vancouver following the Stanley Cup hockey loss to the Boston Bruins, a team of foren­sic specialists spent 24 hours a day for two weeks to process about 5,000 hours of video. The police, who had only charged one suspect before they examined the video, eventually charged more than 300.