Boston Marathon: What Was the Bomb?

Officials recover part of a "pressure cooker" IED.
3:23 | 04/17/13

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Transcript for Boston Marathon: What Was the Bomb?
Police have received 2,000 tips in the boston bombings, that's the scene right there. Investigators combing through it inch by inch. The boston police commissioner says he's never seen a more complex crime scene. And let's show you this again. You see this bag. It's right next to the mailbox right near the finish line. That second photo taken at the exact same spot just after the deadly explosion. This is the kind of thing police are looking for. Any clue that can help them solve who was behind the blast. But there are no suspects right now. Abc's chief investigative correspondent brian ross is right there on the scene reporting the story all night long. Good morning, brian. Reporter: Well, good morning, george. This is a fast-breaking investigation with a major breakthrough already. The recovery by the fbi of the mangled partial remains of one of the bombs that apparently failed to fully explode all made of a common kitchen item. Crime scene technicians discovered the key components along the race route and sent them directly to the fbi crime lab in virginia. These photos obtained by abc news show what was once a $100 medium-sized pressure cooker, twisted from the blast after being turned into a homemade bomb packed with tiny nails and ball bearings to maximize the damage to its victims. This is a very similar device TO THE IEDs THAT WE HAVE FOUND Throughout afghanistan used to kill our soldiers. Reporter: The fbi says the pressure cooker bombs were hidden along the race route in black backpacks. The tattered remains of one of them seen in this photo. Agents want to know if they saw someone carry an unusually heavy bag. Someone knows who did this. Cooperation from the community will play a crucial role in this investigation. Reporter: Among those who have come forward with pictures that could help the fbi is boston tv station whdh which broadcast first this shot taken by a viewer near the finish line, note the large sack in front of the metal fence next to the mailbox. And then this shot, the station says was taken just after one of the bombs ex-he employeded. The area was blurred because of the graphic nature of the material but the viewer said the large bag had disappeared the fbi says anything that stands out is worth a second look including these shots made seconds after the blast by ben thorndike from his office overlooking the finish line. I was struck by this individual everyone else in the photo is on the ground bent over holding their ears appearing to be in shock, immobile but this one individual is exactly the opposite. He's sprinting. His clothes were in tattered. They were melted and burned off his body so for all I know he was panicked and fleeing. Reporter: At this point there's no indication that that man had anything to do with the bomb blast but the fbi now is most interested in finding out who bought two of these pressure cookers. We bought this one, the exact same precise model used by the bomber last night just a half mile away for about $140. A thousand of them are sold every year made by a company in canada and that's a key part. More from richard clarke, the head of counterterrorism for

This transcript has been automatically generated and may not be 100% accurate.

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