'5 Days': The Hunt for the Boston Marathon Bombers

FBI agents reveal how they pieced together the puzzle to find the killers.
9:18 | 04/19/16

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Transcript for '5 Days': The Hunt for the Boston Marathon Bombers
As more than 30,000 runners made their way through Boston. Moments of hope and victory seen different. Tonight deep inside the investigation with dramatic video and new details about how the man hunt ended in a massive shoot out in one man's backyard. Here's Brian Ross. Reporter: It was a joyous day in Boston until that first blast at 2:49. What the hell was that? Reporter: Heads turned at the sound of the explosion. Oh, my god. Something blew up. Something happened. Reporter: Over the next 12 seconds some will decide to leave the restaurant area, but most will stay, including the Richard family, unaware of the second bomb that's planted in a backpack at their feet. Reporter: Among the dead, eight-year-old martin Richard. What should have been joy turned into panic and heart wrenching fear of the worst. I saw the fear in people's face. I seen people crying and leaving the scene very much terrorized. I could see it in their face. Reporter: It was the beginning of five days of terror. And tonight three years later, the FBI and Boston police reveal how over the next 102 hours they would trek down those responsible for an attack that scarred the nation. I was like, oh my god, we got to get who is responsible for this. It was a puzzle at that point, and we had to slowly piece by piece put it together. Reporter: The most important piece of the puzzle came from what's called the cart lab at the Boston FBI head quarters where agents collected video from along the race route. This is the man they were looking for, but during that first day, could not spot. First slow down, zoom in, play again, no luck. He was hidden in plain sight. Reporter: Only after a spectator sent in this photo on day two did it all come together when agents were able to see the backpack that held the bomb. Right there. Reporter: That's the first time? That's the first time we saw the pack. We said that's got to be the bomb. We start to look around the bag of who could have been responsible or who it belonged to. The man in the white hat. Reporter: They sent out to backtrack and find the bomber in the white hat. Going backwards picked up by the cameras at wall greens. We see him coming here. Reporter: And by the camera by the bank of America. The next piece we get is here at whisky's restaurant. Reporter: White hat seen with another man wearing a black hat. When you saw them here, you knew that was the second person. That's where we have black hat and white hat. Reporter: Over the next two days the case stalled. They didn't know what the men were until day four when the FBI made the fateful decision to go public with the surveillance photos. And the bombers rushed to strike again. Oh, my goodness. All units respond. Officer down. Officer down. All units. There was a radio call that there's an officer down and it turns out it's an mit brother. Reporter: The brothers approached the officer's squad car. They assassinated him in hopes of getting his gun. Reporter: After failing to get his gun, they run away. Carjacking a vehicle and driving toward watertown. The vehicle is moving now, units. They're pinging the vehicle. The vehicle is being pinged. Reporter: They moved in toward dexter. The bad guy is out. Arms fully stretched out. Shots fired at officers. I stood here and said this is what I'm going to do. I jumped up, threw it in drive and I just jumped downright here and I was following it. Reporter: The car is moving? The car is moving and I'm following it and shooting between here and using the car as cover. I emptied my pistol and his pistol ran out of ammunition. He turned and ran down back onto the street, took a left. I chased after him, and I probably left about four or five feet in the air and came down on my shoulders and tackled him. He had eight bullets in him and he was fighting us. I had my empty gun. I was trying to knock him out. I hit him as hard as I could, couldn't knock him out. The younger tsarnaev runs over his brother just missing the officers. All officers respond. We have one body pinned down and another one on the run. Shots fired. We saw tail lights at that point. Reporter: With the younger terrorist on the loose, the entire city was shut down on day five. That applies here in watertown where we are right now, and at this point all of Boston. All of Boston. Everybody off street. Let's go. We were going to house after house clearing them. People were hearing footsteps in their homes. People were running scared. Reporter: After a long day the lockdown was called off and Boston police officers were about to head home until this. What's the address of your emergency? 67 franklin, watertown. Reporter: The home of 66-year-old David hen berry. A 9-1-1 call being heard here for the first time tonight. 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. Hello? Ah. Can you look at the boat without making yourself noticeable? I can look at it right now. I'm outside. Go back inside. I can hear -- Don't put yourself in harm's away. Stay away from the boat. I'm not going near the boat. I'm absolutely not going near the boat. And we were literally the first ones on the boat. I went and said we're already here. I'm in the rear of the yard. I got eyes on the boat. We got the guy. Someone is broken holes in the roof of the house. It looked like he was poking with a gun. Reporter: On their own hundreds of agents and police responded to the scene, guns drawn. And I'm like oh, my god. Shots fired. Shots fired. Reporter: More than 250 shots into the boat itself and dozens more into surrounding homes. This one missed a child's bed. Other rounds smashed into walls. Vases, furniture at the homes of neighbors. Hold fire. Hold fire. I knew we had to stop that firing right away. And right away I'm yelling on my radio and I'm yelling to my guys. Hold your fire, hold your fire. Reporter: Only later did police learn that the man in the boat did not have a gun. Who fired the first shot? We don't know. I can honestly say, none of our Boston guys fired. I have good control over my guys at the scene. Reporter: They finally surrounded the boat waiting for an FBI hostage rescue team to take over. We threw flash gangs and smoke grenades and eventually they negotiated. We have movement in the boat. He just sat up. He is moving, flailing about, quite a bit movement. They were able to talk him and get him to it up in the boat. And then he was taken into custody 100%, that's our guy. Reporter: The younger tsarnaev was injured but survived as medics treated him, he asked what happened to his brother who had since died of his injuries. They told him you'll find out soon enough. Job well done. It was the best feeling I've ever had to be out there and to actually see people clapping for police. It was something that I've never seen. Reporter: Our thanks to Brian Ross for that report. The full episodes of five days, the hunt for the Boston marathon bombers willing released daily across ABC news digital platforms.

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

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