Maggie: Sources tell CBS News that investigators are trying to identify a person, described as a “young white man,” who was standing in the crowd near the scene of the second bombing just before that device exploded. A surveillance camera at a nearby Lord & Taylor department store captured images of the man who was carrying a backpack and talking on a cell phone. Sources say he was wearing a black jacket, grey hoodie and a white baseball cap, which was backwards on his head. Officials say the man, who seemed to be alone, put the backpack on the ground. Then when the first explosion occurred at the finish line, about 100 yards away, he took off. Just a few seconds later, the second bomb exploded near where the man had been standing.
Investigators are now are going through cell phone logs to determine who made calls from that location near the time of the explosions. Sources tell CBS the FBI is working with a list of names of cell phone owners and is trying to match one of them to the unknown man on the surveillance tape.
Forensic experts are now expected to try to use facial recognition software and compare the images from the surveillance camera to photo IDs connected to known cell phone users.
Investigators are now comparing the bombs that were used here in Boston to other explosives used in terror attacks around the world.
Officials say the two bombs that exploded during the marathon were improvised explosive devices, or IEDs – homemade bombs that were meant to inflict mass casualties and cause devastating damage. The devices were filled with BBs, ball bearings and nails.
Sources say the bombs were hidden in black nylon backpacks and housed inside sealable metal pots called pressure cookers. Pressure cooker bombs can help boost the power of relatively small devices by briefly constraining the blast. And when the cookers do explode, they can add large chunks of metal to the shrapnel spray.
Ed Davis: There has been debris recovered from some of the rooftops nearby as well as some of the debris has been embedded in some of the buildings nearby. So that gives you just, kind of, the scope of the power of the blast.
Maggie: The IEDs have been popular with terrorists. Al-Qaeda published a how-to recipe in an online Jihadi magazine. Seven of the bombs were used in a 2006 attack on trains in Mumbai, India.
In 2004 and 2010, the Department of Homeland Security warned law enforcement that pressure cooker bombs could present a threat in the U.S.
Investigators were able to stop a planned attack on Times Square in 2010, and they found a pressure cooker among the bomb components in the back of an SUV.
In Boston, Maggie Rulli, Channel One News.