Officer's Killing Spurred Pursuit in Boston Attack A police helicopter hovered above the scene where an M.I.T. campus police officer was fatally shot late last Thursday. CAMBRIDGE, Mass. - Officer Sean A. Collier was 27, not much older than the Massachusetts Institute of Technology students he watched over as a campus police officer, and he sometimes joined them in a game of darts or Xbox. 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. Wendy Ruderman reported from Cambridge, Serge F. Kovaleski from Boston, and Michael Cooper from New York. Reporting was contributed by Katharine Q. Seelye from Boston, William K. Rashbaum and Erica Goode from New York, and Michael S. Schmidt from Washington.