As we come on the air, this is a country in search of a bomber and there are new clues inside those bombs. At this hour, investigators think one or both of the bombs may have been encased in an... See More
As we come on the air, this is a country in search of a bomber and there are new clues inside those bombs. At this hour, investigators think one or both of the bombs may have been encased in an ordinary metal pressure cooker. It may have been packed with nailed and ball bearings to inflict more injury. And tonight new details are surfacing about those who died, including the 8-year-old member of the little league. While at the white house, the flag flies at half staff, the president has just announced he will head to boston on thursday and our team is standing by to bring you the latest. The latest reporting and we'll start with abc's brian ross who is no boston tonight. Brian? Reporter: No one is in custody, but there are new developments about the bomb and the bomber. Vffingsors at the crime scene have already recovered the remains of one pressure cooker bomb which failed to fully explode. According to officials briefed on the case. Late today the fbi said it's looking for a man carrying an unusually heavy black bag. The bag would have been heavy because of the components believed to be in it. Reporter: The fbi has losts of clues already. The white smoke indicates a bomb built with low velocity mixture turs, not military grade. They may not have had the resources as we've seen in other bomb attacks, butnew how to make the bomb go boom. Reporter: Experts say the large pieces of metal, as seen here in the air suggest the bombs might have been concealed in or under a mailbox or trash barrel, as one witness described. I literally saw the garbage barrel explode. I saw at the flash, the fire, the smoke and I ran as fast as i could. All the evidence they collected might not look like much, but the investigators, every small piece of evidence they collect will be a treasure trove. Reporter: In the fbi belief about the pressure cooker bears out, it will be no surprise to counterterrorism officials who have warned about their possible use for years. There are numerous descriptions online about how to build a pressure cooker bomb, including this detailed one posted in an al qaeda internet publication. April is also a month with significance for domestic terror groups. The month of the waco raid and the oklahoma city bombing. As president obama made clear today, there's still no evidence of who is responsible for this act of terrorism. This was a heinous and cowardly act, and given what we now know about what took place, the fbi is investigating it as an act of terrorism. Reporter: The fbi agents tell us they have tons of video and somewhere in there will be a picture of the person who planted these two bombs, diane. Speaking of pictures, I want to ask you about a photo that everybody was e-mailing today. It is a man standing up on top of a roof. You can move in there, you see it right there. What do you know about it? Reporter: Well, it's very intriguing, but authorities say they're giving it no special priority. It's one of many tips and leads they're following. They're really focusing on the person who had that unusually heavy black bag the day of theerates. As you said the strongest
This transcript has been automatically generated and may not be 100% accurate.