Transcript for The 2016 Boston Marathon Takes Place Under Heavy Security

And now, today's Boston marathon. It went off without a hitch, under heavy security. Three years after those deadly bombings at the finish line. And tonight, we have new details from inside the FBI. Here's ABC's chief investigative correspondent, Brian Ross. Reporter: It was a joyous day in Boston, until that first blast. Oh! What the hell was that? Reporter: This key surveillance video shows the crowd reaction. Oh my god, something blew up. Reporter: Over the next 12 seconds, some will decide to leave the area. But most will stay. I was, like, oh, my god. We got to get who is responsible for this. Reporter: Tonight, the FBI agents who cracked the case using this chilling video, provide new details and show how it was done. He was hidden in plain sight. Reporter: For the first 24 hours, the agents could not pick out the man they were looking for -- this man. Then, this still picture came in on day two, and at the site of the bomb was this backpack, and nearby, the man in the white hat. That was the a-ha moment, this is who we're looking for. Reporter: Putting the surveillance video in reverse, agents then spotted white hat with another man, black hat. But tamerlan and dzhokhar tsarnaev. But they would not be stopped for two more days. Oh, my goodness. Reporter: Only after one police officer was killed -- Officer down. Officer down. Reporter: And a wild shootout with police left one of the bombers dead. The remaining bomber hid for 17 hours in this boat, until the owner discovered him and called 911. I have a boat in my yard, there's blood all over the inside. There's a person in the boat. Are you sure? I just looked in the boat. Reporter: Within a few hours, the younger tsarnaev was in custody and the city and the nation applauded. Had they not been caught, they had planned to head to New York and set off bombs here in times square, George. Hard to imagine it could have been even worse. Thanks, Brian. You're going to have a lot more tonight only "Nightline." And the full episodes of "Five days" will be released daily across ABC news platforms. And in Boston today, more than 30,000 runners crossing that finish line attacked three years ago, under sunny skies. So many inspirational stories. This picture showing two runners carrying a third runner across the finish. And at fenway park, Jeff Bauman, a double amputee, seen here in that iconic photo the day of the attack, throwing out the first pitch, right down the middle.

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