The following is a script from "Manhunt" which aired on March 23, 2014. Scott Pelley is the correspondent. Robert Anderson and Pat Milton, producers.

The two explosions that tore through the Boston Marathon nearly a year ago were like a starting gun on a second race against time. Unknown terrorists were on the loose and they had more bombs. Now, for the first time, you're going to hear the inside story from the federal investigators who ran the manhunt. They led a taskforce of more than 1,000 federal agents, state police and Boston cops.



Tonight, they will speak of the disturbing evidence that cracked the case and of a debate among the investigators that ultimately led to the dragnet's violent end. The afternoon of April 15th, the FBI's man in charge of Boston got a text, "two large explosions near the finish line." For Special Agent Rick DesLauriers, the marathon became a sprint to catch the killers before they struck again.

Rick DesLauriers: I felt that week that I had the weight of the world on my shoulders. And I'm sure I wasn't the only one.

Scott Pelley: Did you feel if there was a third bombing, it would be on you?

Rick DesLauriers: That's everybody's fear. It would be on me. It would be on the Joint Terrorism Task Force. But that wasn't what drove us. What drove us was preventing more people from getting hurt.

264 were hurt already. Three killed. Moments before, Rick DesLauriers had been counting the days to retirement, 26 years at the FBI. Now, on Boylston Street, he took over the biggest investigation of his life.

"I felt that week that I had the weight of the world on my shoulders. And I'm sure I wasn't the only one."



Rick DesLauriers: It was a scene of devastation. There was evidence everywhere.

Scott Pelley: And you said what to yourself in that moment?

Rick DesLauriers: I said, "We will find those responsible for these despicable crimes."

FBI Director Robert Mueller ordered every office in the world to back up DesLauriers. DesLauriers' link to headquarters was Executive Assistant Director Stephanie Douglas.

Stephanie Douglas: We were very, very concerned about other bombs in Boston. But we had to think beyond that. Were there other bombs in other cities? Were U.S. interests even abroad at risk? So we had to consider everything. We could not eliminate anything.

First, came the crime scene -- 12 blocks of debris, abandoned backpacks, and bomb parts blown to smithereens.

Rick DesLauriers: I said, "We will find those responsible for these despicable crimes."

They set up a grid pattern. Evidence could be on windowsills. Evidence could be on roofs of buildings. Evidence could be anywhere.

"We were very, very concerned about other bombs in Boston. But we had to think beyond that. Were there other bombs in other cities? Were U.S. interests even abroad at risk? So we had to consider everything. We could not eliminate anything."



Scott Pelley: Are they going down the street with tweezers?

Stephanie Douglas: Sometimes. When they need to they're doing that. But yes, they are very carefully picking up everything they see.

They saw a battery pack for model cars and chunks of two pressure cookers. The cookers concentrate the explosion for maximum force.

Scott Pelley: You decided to set up a warehouse near Logan Airport.

Stephanie Douglas: Right.

Everything swept from the street was processed in this 46,000 square foot warehouse. Twice a day a plane flew the items to the FBI lab in Quantico, Va.

Stephanie Douglas: And you basically have almost like an assembly line of evidence. So it gets tagged, it gets recorded as evidence so that you're preserving that chain of custody. But it also-- for all the different parts and components that would later go to recreating the devices themselves.

Scott Pelley: The bombs.

Stephanie Douglas: Yes.

Scott Pelley: Recreating the bombs.

Stephanie Douglas: Yes.

Scott Pelley: From the pieces that you found.

Stephanie Douglas: Yes.

Successful as that was, it turned out, the evidence that would solve the case had been collected before the first bomb exploded that Monday. The FBI could travel back in time though the lenses of dozens of security cameras up and down Boylston.

Stephanie Douglas: Almost 13,000 different videos were obtained and 120,000 actually more than 120,000 still photographs.

At the FBI lab in Virginia, 120 analysts were searching video feeds from Boston.

Scott Pelley: What are you looking for?

Rick DesLauriers: Somebody who just doesn't look similar to others in a crowd who would be watching a race.

Scott Pelley: Was there a Eureka moment in terms of the video? At some point somebody said, "Hey, boss, have a look at this"?

Rick DesLauriers: Yes, there was. It was I believe, early Wednesday morning. And we watched that video hundreds and hundreds of times.

Stephanie Douglas: You can see an individual, a tall man wearing a white ball cap walk into the frame. He has a backpack slung over one of his shoulders. He puts the backpack down very nonchalantly. He joins the crowd. You clearly see everybody look very, very definitely to the left like they've heard something. They've seen something. So you know that first blast has gone off. He does not do that. He does not do what everybody else in that video does, he does not turn to his left. He instead just stands there for a second or two and walks very deliberately back the same direction that he came in.



The Eureka video hasn't been seen by the public. It is being kept for the trial. But this still photo shows much the same view of the suspect and the people who would be torn apart by the blast.

Scott Pelley: Let me ask you to describe what you see in that picture.

Rick DesLauriers: I see the subject, the individual who has been charged in the investigation. And I see people who are grievously, who are grievously injured in that blast. And I see individuals who died in that blast.

Scott Pelley: The people along the fence line there.

Rick DesLauriers: Several of them, yes. Very, very emotional time when I look at that, to know what happened a few moments afterwards.

Stephanie Douglas: I believe I see his backpack on the ground. And then I see one of the people that was killed as a result of that bomb.

Scott Pelley: Do you know his name?

Stephanie Douglas: It's Martin Richard.

Martin Richard was 8 years old. His 7-year-old sister, Jane, lost a leg. Their father, Bill, suffered hearing damage from the bomb, in the backpack, laid at their feet. In the video, the backpack explodes 20 seconds after the man in the white hat walks away. Stephanie Douglas saw it in the FBI's Washington Command Center.

Scott Pelley: Nobody I've talked to can quite find the words.

Stephanie Douglas: It's a horrible video to watch. I mean, after you, after the bomb goes off, obviously it's a very smoky situation. There's a lotta smoke. And the smoke clears. And what you saw is very happy scene of people watching that marathon is no longer that. Even after seeing something so horrible I remember this survivor who unfortunately, his clothes were on fire. And I just remember this police officer getting down on his hands and knees and putting out the flames on this person with his bare hands. And I just thought to myself, you know, "What an incredible contrast of events. Something so horrific and then we have this person with no thought or of his own comfort or consequences to himself rush in and actually do something like that. That was so brave." I still remember that very clearly.

Only two days had passed. Now they were looking for every image of the suspect they called "white hat." Massachusetts state police analysts found him with a man in a black hat.

Scott Pelley: Which turned out to be his older brother.

Stephanie Douglas: Yes.

Scott Pelley: Now you have the Tsarnaev brothers.

Stephanie Douglas: Yes.

Scott Pelley: But you don't know that.

Stephanie Douglas: No, I don't know that. I don't know who they are.

Then suddenly that Wednesday confusion reigned when cable news channels erroneously reported that a suspect had been arrested and was headed to the courthouse. The error caused pandemonium according to U.S. Attorney Carmen Ortiz who's leading the prosecution.

Carmen Ortiz: And I remember turning to my colleague, and saying to my press person, saying, "Do we have someone in custody?" And I turned to Rick DesLauriers, "Do we have someone in custody?" And-- and they were like, "No, we don't have anyone in custody." I think that kind of misinformation makes it appear as if government isn't in control, the investigation is sort of, you know, confusing. And so it can be very, very harmful.

More harm was done the next morning when the New York Post added to the erroneous reporting by putting a man with a white hat on its front page.

Scott Pelley: The problem was it wasn't the right man in the white hat.

Carmen Ortiz: That generated tremendous risk and harm. It gives people a false sense of security thinking, "Oh they've identified these suspects" when it turns out that it's wrong individuals. It puts those individuals at tremendous risk.

The risk to the innocent lent urgency to the debate over whether to release the real pictures.

Scott Pelley: Why wouldn't you release the pictures? Isn't that the fastest way to find the perpetrators?

Stephanie Douglas: Sure. But it also gives them every opportunity to escape. Remember, we do not have the identities of anybody.

Scott Pelley: So your concern was that if you put the pictures out there to the public they'd know they'd been had, and they'd run?

Stephanie Douglas: Yes, absolutely.

Rick DesLauriers: The countervailing argument is you had individuals, we had photographic evidence of individuals, who we strongly believed were responsible for the bombings and we need to identify them as quickly as possible.

So Thursday, Rick DesLauriers walked out in time for the evening news.

[Rick DesLauriers: These images should be the only ones--I emphasize the only ones--that the public should view to assist us.]

The pictures set events in motion that DesLauriers didn't predict, in fact, didn't recognize even after they started.

Rick DesLauriers: My wife was watching the news that evening.

[Breaking News: An officer has been shot...]

Rick DesLauriers: And there was a story about an MIT police officer who was subsequently identified as Officer Sean Collier who had been murdered that evening and right on campus. My wife looked at me and she said, "I bet those are your guys and they're on the run right now. And I bet they murdered this police officer." And I didn't believe her. I said, "Oh, no, I don't think so." And I went to bed.

Scott Pelley: Your wife had cracked the case and you went to bed.

Rick DesLauriers: I did not believe that she had cracked the case at the time. I went to bed.

Stephanie Douglas: I went to bed probably around 10:00 o'clock. And, this is probably a sad commentary in my life, but my Blackberry was on the pillow next to me...

Rick DesLauriers: And somewhere around 12:30, quarter to one in the morning, I received a phone call from one of my assistant special agents-- agents in charge, Jeff Sallet...

Stephanie Douglas: And my phone rang about a little after 1:00 in the morning.

Rick DesLauriers: I woke her out of a sound sleep. And I said, an MIT police officer has been murdered earlier this evening by individuals we believe to be responsible for the bombing. And they are on the streets of Watertown right now engaged in a shootout with the Watertown Police Department.

It was combat. Two suspects threw pipe bombs and a pressure cooker bomb at the police. An officer was gravely wounded. Those who argued that releasing the pictures would cause the suspects to run--were right.

Scott Pelley: Was putting the pictures out the right call?

Stephanie Douglas: Yes, I think at the end of the day, we really had no choice. Believe me, the death of Sean Collier is not lost on the FBI. We consider it an incredibly tragic event. But I think at the end of the day, given the facts as we knew them at the time, we made the best decision.

Scott Pelley: How do you feel about that decision now?

Rick DesLauriers: I stand by that decision, Scott. Nobody could have reasonably foreseen that a police officer would be murdered. What could reasonably be foreseen is that these individuals could have had more bombs could have set those bombs off and caused carnage similar or even greater to than what they caused on April 15th.

One suspect was killed, the other vanished. In minutes the FBI matched the dead man's fingerprints to Tamerlan Tsarnaev an immigrant from Kyrgyzstan. Other records showed he had a brother.

Stephanie Douglas: Yeah and I remember that so clearly. Somebody walking in with a manila folder and said, "OK, here's his brother." And they opened it. And it's his picture. And I go, "That's him. That's white hat. That's who we should be looking for."

Friday, the governor ordered a lockdown of Boston. But a house to house search turned up nothing.

Stephanie Douglas: Everybody's exhausted and deflated. You know, I mean, it's a very sad day for Boston, another sad day for Boston.

But then, a man noticed someone in a boat in his backyard. Dzhokhar Tsarnaev was wounded but alive.

Scott Pelley: What did you think as he went into court to be arraigned?

Rick DesLauriers: He had a smug grin on his face much of the time in the courtroom. He would glance over his right shoulder back to his relatives, and smile, and smirk at them. And I found that absolutely galling. And I found it reprehensible.

Scott Pelley: The attorney general has decided to seek the death penalty in this case.

Rick DesLauriers: Yes, he has. And I support that decision.

Dzhokhar Tsarnaev has pled not guilty. His defense team declined to speak with us. U.S. Attorney Carmen Ortiz is preparing for a November trial.