So when an ambulance staffed by students rolled past his parked patrol car last Thursday night, he flashed his blue lights to say hello. The students answered with their red lights.
It was just a little after that routine interaction, the police said, that a pair of men approached Officer Collier’s squad car from behind and shot him to death, in what some law enforcement officials said appeared to have been a failed attempt to steal his gun. In the anguished scene that followed, the student emergency medical technicians were called back to the patrol car they had just passed, where they tried in vain to save Officer Collier’s life.
The killing of Officer Collier, who was mourned Wednesday at a campus memorial at which Vice President Joseph R. Biden Jr. spoke, was the first bloody altercation in a nearly 24-hour chain of violent events that left one of the brothers suspected in the Boston Marathon bombings dead and ended with the capture of the other.
Interviews with law enforcement officials and witnesses painted a clearer picture of what happened during that chaotic period, and correct some of the information that officials gave out as they hunted the most wanted men in America.
Police officials initially announced that officers had “exchanged gunfire” Friday evening with the surviving suspect, Dzhokhar Tsarnaev, 19, as he hid in a boat in the backyard of a house in Watertown, Mass. Now several law enforcement officials say no gun was found in the boat, and officials say they are exploring what prompted officers to fire at Mr. Tsarnaev, who some feared was armed with explosives.
Law enforcement officials now say they have recovered only one gun elsewhere, which they believe was used by Mr. Tsarnaev’s older brother, Tamerlan — not the three previously reported. And initial reports that the brothers first came to the attention of the police after robbing a 7-Eleven were wrong. The police were called to a gas station convenience store early Friday after a man who said he had been carjacked by the marathon bombers escaped and sought help.
The catalyst that set the violent night in motion was the shooting death of Officer Collier, officials said. It came about five hours after the F.B.I. released pictures of the two suspects in the bombings and asked the public’s help in identifying them.
“I consider him a hero,” Boston’s police commissioner, Edward Davis, said in an interview this week. “It was his death that ultimately led to the apprehension. The report of the shot officer led to all those resources being poured in.”
Officer Collier was killed around 10:30 p.m., police officials said — just half an hour before his 3-to-11 shift was to end.
While there is video of two men approaching Officer Collier’s car, three law enforcement officials said, it does not clearly show their faces. But investigators now believe the brothers killed the officer to get another gun.
“He had a triple-lock holster, and they could not figure it out,” a law enforcement official said. “There is evidence at the scene to suggest that they were going for his gun.”
The killing brought a huge influx of police officers into Cambridge, so plenty of officers were in the area later that night when a 911 call reported a carjacking by two men claiming to be the marathon bombers.
The two men apparently split up after the killing, and when the carjacking occurred, before midnight, a lone man approached a parked Mercedes-Benz sport utility vehicle and tapped on the passenger-side window, officials said. Why the men separated was among the many details of the night that were still unclear even a week later.
After the driver lowered the window, the man reached in, opened the door, climbed in and pointed a gun, saying, “Did you hear about the Boston explosion?” and “I did that,” according to an affidavit filed Monday with the criminal complaint charging Dzhokhar Tsarnaev with the bombings.
The gunman, who law enforcement officials believe was Tamerlan Tsarnaev, 26, removed the magazine from his gun and showed the driver that a bullet was in it, according to the affidavit. “I am serious,” he was quoted as saying.
The gunman then forced the driver to head for another location, where they picked up a second man, who officials believe was Dzhokhar Tsarnaev. The brothers loaded something in the trunk of the S.U.V., the affidavit said. They then took $45 and a bank card from the carjacking victim, it said.
They pulled into the Memorial Drive Shell gas station and Mary’s Deli food mart at Memorial Drive and River Street in Cambridge after midnight. Video surveillance shows the victim of the carjacking fleeing to a Mobil gas station across the street, according to Alan Mednick, the Shell station’s general manager. The brothers did not buy anything, Mr. Mednick said.
“Maybe they were going to try and buy something, but the guy took off running,” he said.
At the Mobil station, Tarek Ahmed, 45, was working the overnight shift when, he said, a panicked man “came in running.”
“He opens the door,” Mr. Ahmed recalled in an interview. “I stood up. He was screaming, saying: ‘Call the police. They have bombs. They have a gun. They want to kill me.’ I thought he was drunk.”
Then Mr. Ahmed realized he was serious.
“He ran behind the counter and ran into the back room, a storage room, and locked the door,” Mr. Ahmed recalled. “At this moment, I believe him. He was honest, that somebody wanted to shoot him. So I took the phone, and I called 911.
“I tried not to look outside at anything. I wanted to make it appear as if nothing was wrong. I was hoping the suspects didn’t see where he went. At the same time, I told the police what happened. As I’m talking to the police, I back up slowly and knock on the storage room door. The guy opened the door, and I handed him the phone.”
The carjacking victim left his cellphone in the Mercedes, a law enforcement official said, allowing officials to track it.
They caught up with the S.U.V. in Watertown, where the men “threw at least two small improvised explosive devices” out of the car, the affidavit said.
A furious gunfight ensued on Laurel Street in Watertown, where more than 200 rounds were fired, officials said. A transit police officer, Richard H. Donohue, was shot in his right leg and critically wounded during the gunfight.
Chief Edward P. Deveau of the Watertown police said the suspects were shooting at seven of his officers on a side street and throwing explosives at them.
“One of the officers coming in had at least one bullet go through his windshield, and had the wherewithal to put the car in gear and let it roll down the street while he is able to get out and take up a position,” the chief said. “And eventually it hit a parked car. They were shooting at it because they think there is somebody in it.”
Tamerlan Tsarnaev was wounded in the gunfight and then, law enforcement officials and witnesses said, was hit by the carjacked Mercedes, which his brother used to escape.
Mike Doucette, 27, a chimney sweep who lives on the street, described seeing one brother shot and fall to the ground. He was still moving when the other brother went “screaming up the street” in the S.U.V.
“I yelled to the cops, ‘Watch out!’ ” Mr. Doucette said in an interview. But the car hit the wounded brother, he said, and “his body was tumbling underneath.”
As Friday dawned, state officials urged people in the Boston area to stay behind locked doors, and all transit service was shut down — paralyzing the metropolitan area as officials searched for Dzhokhar Tsarnaev. That evening they lifted the order, fearing he had escaped.
Then, close to 7 p.m., they got a call from a Watertown man. The man, Dave Henneberry, had stepped out of his house after the authorities gave residents the all-clear, then noticed something askew with his boat and went to fix it, said John Duffy, 66, a close friend. The boat was called the Slip Away II.
A little later, Mr. Henneberry decided to return to the boat. “So he brought a ladder out of the garage and laid it against the boat,” Mr. Duffy, who has talked to Mr. Henneberry several times about the episode, said in an interview. “He took about three steps up the ladder and looked inside the boat and sees blood on one side of the deck. And he is questioning himself, ‘Did I cut myself the last time I was out here?’
“Then he sees blood on the other side of the deck,” Mr. Duffy said. “Then he looks over the engine compartment and sees a body. His words were, ‘I levitated off the ladder.’ He does not remember going back into the house. He told his wife, ‘Lock the doors,’ and he called 911.”
A call went out over the police radio.
“They have a boat with blood on it, and they believe someone’s on the boat,” it said.
Police officials initially said the boat was in the backyard of a house just outside the perimeter of the area where investigators had conducted door-to-door searches all day.
But Commissioner Davis, of the Boston police, said this week that the boat had been inside the perimeter.
“It was an area that should have been checked,” he said.
“We are not sure how long he was in the boat.
There was a pool of blood near where the car was dumped about four or five blocks away from the boat.”
It is still not clear what prompted officers to fire into the boat.
“Shots fired, multiple shots!” someone was heard saying on the radio, before another call went out: “All units hold your fire! Hold your fire.”
Commissioner Davis said that “we will have to see what prompted the volley of shots before the cease-fire was ordered by a superintendent of the Boston police.”
A state police helicopter used thermal imaging technology to show where the suspect was hiding in the boat, and a robotic arm attached to a police vehicle was used to pull the tarp back.
Then, around 8:45 p.m., Dzhokhar Tsarnaev was taken alive.