It was a 7-Eleven's security footage that helped trace Tamerlan and Dzhokar Tsarnaev to a Boston suburb the day after the U.S. Federal Bureau of Investigation publicly released photos and videos of the Boston Marathon bombing suspects. But that won't be the end of video evidence in this case. Now that Dzhokar has been captured, U.S. agencies will comb through thousands of hours of video captured on security cameras, people's cellphones and other sources, looking for evidence to build their case against Tsarnaev in court, Grant Fredericks tells Popular Science. Fredericks is a forensic video instructor at the FBI National Academy. He also manages the use of the National Multimedia Evidence Processing Lab in Indiana. It's an enormous task, but some technologies, both old and new, now make it a little easier. I talked with Fredericks about the tech tools he and his colleagues at the FBI use. The following comes from phone and email conversations last week and today.--Francie Diep

Police agencies are beginning to realize that the most prolific source of evidence available to law enforcement comes from video images. Police agencies are recovering more and more video at everyday crime scenes. More than they did a year ago, more than they did the year before.

They have to be taught about what they're looking for. But through the algorithms, they can be taught to examine crowds of people and identify people with backpacks.