Dateline   |  April 24, 2013

The Hunt for Answers, Part 2

Following the release of video of two suspects, violence breaks out in Cambridge.

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This content comes from Closed Captioning that was broadcast along with this program.

>>> with the fbi's release of photos and videos of the two suspected bombers, the 24-hour news cycle launched into high gear . the hunt was on.

>> now we know who they are looking for.

>> these are the suspects.

>> who are these two men?

>> their faces were plastered everywhere. a stream of tips pouring in. and the walls began closing in on the suspected bombers.

>> once we saw their picture show up on tv, they went into panic mode.

>> nbc news terrorism analyst aaron kuhlman believes the suspect's blown cover thrown them into a tailspin. while the bomb was executed with precision, now it appears they have no plan of escape.

>> they started making grievous errors, in retrospect, i don't think you could possibly explain. i don't think they were the product of rational thinking.

>> as darkness fell on boston last thursday, the entire region was still on edge. no one knew where the suspects were, what they were planning next, or when they would resurface. m.i.t. campus police officer sean collier was on patrol in cambridge around 9:30 p.m . when his boss, john difava spotted him.

>> i pulled up next to him in the cruiser. i said what are you doing? just watching traffic, chief. keeping my eye on things.

>> one hour later, the chief got the call, officer down. collier had been fatally shot in his patrol car .

>> my reaction was total disbelieve. i know i have 57 brokenhearted police officers . we have his family who is devastated and a community that, you know, obviously from the outpouring of sympathy and support, they are devastated too.

>> why ambush collier? the shooting continues to confuse and baffle cambridge police commissioner robert hawes, who gave "dateline" a detailed account.

>> it appears somebody or somebodies approached from behind collier's vehicle and fired four or five rounds into the vehicle, striking officer collier each and every time for the number of shots we could determine.

>> did the suspects think officer collier had recognized them? or perhaps even though they had at least one gun, they wanted his weapon too? boston police commissioner ed davis .

>> why ded they pop up back on the radar? why did they kill officer collier?

>> it is hard to say. it goes to part of the investigation that is impossible for me to talk about. for some reason they activated themselves, started to kill again, and they were heavily armed, so i think people have to draw their own conclusions at this point in time as to why they did that.

>> and the violence wasn't over yet. midnight when police responding to cambridge , there was a carjacking not far away.

>> too many things in close proximity for this to be coincidences.

>> it was the bombing suspects and they took their carjacking victim on a terrifying ride. he later told investigators that one of the carjackers pointed a gun at him and said did you hear about the boston explosion? and i did that. the suspects also demanded cash, and drove the victim to three different atms. here is a surveillance photo of dzhokhar at one of them. the next stop, and what appeared to be an improvised flight was this gas station .

>> they pulled into a gas station to refuel, his car was about out of gas, he saw his chance and escaped.

>> justice correspondent pete williams has been leading coverage of the investigation. he communicated with the carjacking victim via e-mail earlier this week.

>> he described them as brutal, but cautious and said he was in fear for his life.

>> the carjacking victim escaped by running from the gas station to another station across the street. and asked attendant tareq ahmed no help.

>> he said, please, please call the police. someone wants to shoot me. they have the bombs, they have guns. they took my car. they -- they want to kill me.

>> the suspect's plan, whatever it was, was unraveling. the calm demeanor known by authorities in the surveillance video, seemed to be replaced with panic and desperation.

>> they acted in a very impetuous and irrational manner, but that may also indicate that they didn't know what to do with that point. their plan, their plan had come to an end and they didn't know what their next step would be.

>> the carjacking would prove to be their fatal mistake, undone by police with help from a cell phone. a citizen, and modern technology