Investigators say they've identified a "number of people" they're interested in talking with.
Investigators continue to pour over evidence they've gathered from the scene of Monday's bombing near the Boston Marathon finish line.
According to NBC News sources, the FBI has also gathered leads based on photos and security videos taken along the race route Monday.
Sources tell NBC News a significant break in the case came after reviewing hundreds of hours of video tape, including security video from a department store that captured images of a man leaving a bag near the second blast sight.
Bomb teams focused on the crime scene near the finish line throughout the day Wednesday and uncovered what's been described as "solid leads" in the investigation.
Also a new series of photos have surfaced that show remnants of one of the bombs used in the attack and pieces of shrapnel apparently packed into that device.
Outside the crime scene police and soldiers with assault weapons stand guard across the city, part of a stepped up security presence that authorities say will continue for the foreseeable future.