Boston Marathon Explosions: Inside the Investigation

ABC News? Pierre Thomas breaks down the latest from law enforcement.
3:00 | 04/15/13

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Transcript for Boston Marathon Explosions: Inside the Investigation
It's been more than eleven years since the 9/11 attack shocked this country into the 21 century and since then there have been more attempted attacks the shoe bomber the underwear bomber the Times Square bomber. Plots foiled plots we never learned. Over the years as the country's homeland -- -- hardened the fears seem to abate. Until today. And now the attention focuses on those two issues who did this. Bring them to justice with the latest on the investigation here's ABC's Pierre Thomas. It is dramatic videos like this. And this -- at the scene of the horrific blast today but investigators will be -- -- for -- of what happened this afternoon. Two bombs exploding within seconds of each other and only a hundred yards apart. The bombs was small portable devices that could have been carried in the backpack. Easily concealed among the thousands of spectators. And that's what makes this investigation so typical. Doesn't take a lot of expertise to make small hand held her backpack sized bombs. You can go to the -- Internet at this moment. In it we'll tell you the component. -- will tell you about timers be able to tell you about using cellphones to detonated it as one device. Authorities say there are no suspects but tonight congressman Peter King tells ABC's Denzel and there's a person at a local hospital who's come under law enforcement scrutiny. I've heard several reports that there's a person who is incarcerate. He is being questioned. And -- this -- were described as a person of interest right now. The bombs caused terrible injuries and hospitals reported seeing shrapnel wounds. There is -- a lot of small metal debris Tom. Some people have have passed already about whether this word. BBA's are there -- parts of bombs and I just don't think we're able to say whether it is -- small bits of metal that were placed there intentionally or rather they were just sort of environment. The shrapnel was just one of the clues investigators will be looking. There's fragments everywhere obviously there are fragments within the victims there's going to be -- and sidewalk. And with the going to be able to do. Is probably reconstruct. Indeed -- timer device what was used they're gonna look at the components of the bomb. It will tell them assistant somebody that put this together over the Internet or is this a bomb maker. Hundreds of state and federal investigators have descended on Boston to try to find out who could have built the bombs. And set them off. -- federal state and local law enforcement. All on seen in coordinating. Very closely the FBI has taken charge of the investigation. This type of attack has long been law enforcement worst fear a soft targets were suspects can. -- -- -- and inflict mass casualties police. Fortunately these kinds of settings whether it's a marathon football baseball social event concert. Are attractive -- you just can't get. Boston Marathon is a heavily policed event. More than 500 National Guard troops were on scene this afternoon as well as members of the Boston Police and fire departments. That did not prevent this from happen. Anyone can walk up to the sidelines. And that stretch of Boylston street. Anybody can get off the subway. Or bicycle or walk down. And walk right up to lines where this -- took place. It would be possible. To screen everybody. Along the 26 mile route of -- Cities around the nation now on high alert. Bomb sniffing dogs patrolling the streets of Chicago. -- -- in Washington and New York stepping up their security. In New York the -- the police presence could be seen everywhere. Police cars lined the streets in Times Square which itself was the scene of an attempted bombing in 2010. That was only foiled when a local vendor alerted police about a suspicious car. Today's attack is one of many targeting US civilian. In 1993. Terrorist targeted the World Trade Center killing six people. Two years later in 1995. Was the Oklahoma City bombing 168. People killed in that attack. Similar to today's attack in Boston the bombing of the Atlanta Olympics in 1996. Spectators gathered in the park when the attack took place. And the horrific events of 9/11. Thousands killed still seared into memory. Tonight investigators are just starting to comb through the evidence collecting the security camera video from the scene searching for any clues. About who could have done this. And -- The FBI and Boston Police. Massachusetts state police will be going over all videotapes from that entire neighborhood. Looking to see if there was someone suspicious. There was someone may recognize. Doing facial recognition analysis and running facial software could see. Who was there were Nightline on Pierre Thomas in Washington.

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

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