Police believe Tsarnaev brothers killed officer for his gun

(CBS News) -- A remaining mystery of the Boston Marathon bombings manhunt has been why Sean Collier -- a campus police officer at MIT -- was apparently lured into an ambush and shot to death on Thursday night.

Police now think they have the answer. Investigators now believe that Officer Collier was killed because the two bombing suspects wanted to take his gun.

Tamerlan and Dzhokhar Tsarnaev only had one real gun and one pellet gun. Investigators believe because the officer's holster had a locking system, they apparently couldn't get the gun out.

Officer Sean Collier was shot in the head execution-style while sitting in his patrol car.

One of the first responders was officer Richard Donohue. A few hours later, he would be seriously wounded in a shootout with the Tsarnaev brothers.

Tamerlan Tsarnaev died in the gun battle. The hunt for Dzhokhar Tsarnaev came to a head the next evening when a man spotted the 19-year-old hiding under a tarp on his boat in the backyard.

Four members of the Boston Transit Police SWAT team, Kenny Tran, Brian Harer, Saro Thompson, and Jeff Campbell got the call to apprehend the suspect.

Jeff Campbell: I saw the suspect sitting up on the -- I believe it's called the gunnel of the boat with one leg hanging over the side and his hands were just sort of down by his side. He looked weak. He was shaky, appeared to be losing consciousness and did have some wounds to his body. You could -- you could see the blood on his body.

John Miller: And what are you telling him to do?

Campbell: We're, we're commanding him to get down off the boat. We don't know if he has a weapon or some type of explosive ignition switch that he could just reach down and hit. We waited to a point where, as we were getting closer to him, and both hands came up, we saw that both hands were open. We bolted out in front of the shield at that point. It was the one chance we had and we wanted to end this for everybody.

Miller: I'm wondering what it's like when you're in the tactical mode. You think, "I have a suspect. He may have a bomb on him"

Kenny Tran: Well, just -- your training kick in. You know, you just -- you can't be scared. You just gotta do the task at hand.

Miller: What was it like to be the person who put the handcuffs on probably the most wanted man anywhere?

Saro Thompson: At that time you don't think about that. I just put the cuffs on, brought his hand around. Somebody else, holding the other hand. These guys were helping me, along with other people. We cuffed the other hand and we just moved out.

It appears as if Dzhokhar Tsarnaev did not have a gun in the boat. So how did the shooting start there?

The issue goes to what the police were perceiving. One officer had the high ground and saw the tarp on the boat flip up and said he saw a hand come out with an object in it. He didn't think it was a gun, he thought Tsarnaev was about to throw another bomb. He fired a couple times.

Other officers who were just arriving, saw the tarp flip up and heard gun fire and apparently believed it was Tsarnaev firing out, so they laid down covering fire at that boat until the incident commander said cease fire.

John Miller

John Miller is a senior correspondent for CBS News, with extensive experience in intelligence, law enforcement and journalism, including stints in the Office of the Director of National Intelligence and the FBI.