Other law enforcement officials said the shooting may have been prompted by the chaos of the moment and some action that led the officers to believe Tsarnaev had fired a weapon or was about to detonate explosives.
These new details emerged as investigators continued their examination of the movements and motives of Tsarnaev, 19, and his brother, Tamerlan, in last week’s coordinated bombing, which killed three people and wounded more than 250.
Law enforcement officials said they do not believe the brothers were connected with a terrorist organization, but they cautioned that the inquiry is at an early stage.
Tamerlan Tsarnaev, 26, was killed in a confrontation with police in the early morning hours Friday, four days after the marathon bombing. A transit police officer was seriously wounded in the exchange, in which more than 200 rounds were fired and the suspects lobbed homemade explosives at police. Dzhokhar Tsarnaev escaped and was the subject of a massive manhunt. He was cornered hiding in a boat in the driveway of a house in Watertown, Mass., on Friday evening.
Law enforcement officials described the 30 minutes before the arrest of Tsarnaev as chaotic. One characterized it as “the fog of war” and said that in a highly charged atmosphere, one accidental shot could have caused what police call “contagious fire.”
Officers from several agencies gathered around the Watertown house as darkness fell. The FBI was in charge of the scene, but there also were officers from the Massachusetts State Police, local police and transit police.
“They probably didn’t know whether he had a gun,” said one law enforcement official, who like others interviewed for this article spoke on the condition of anonymity to discuss an ongoing investigation. “Hours earlier, he and his brother had killed a police officer, shot another officer and thrown explosives out of their cars as the police were chasing them. They couldn’t assume that he did not have a gun and more explosives.”
The FBI declined to discuss the exact sequence of events that led officers to open fire on Tsarnaev’s hiding place and whether the dozens of bullets that struck the boat caused any of his gunshot wounds.
A spokesman for the FBI said law enforcement agents were tracking an extremely dangerous suspect who had used guns and explosives on a public street to avoid arrest.
“Law enforcement was placed in an extraordinarily dangerous situation,” said FBI spokesman Paul Bresson. “They were dealing with an individual who is alleged to have been involved in the bombings at the Boston Marathon. As if that’s not enough, there were indications of a carjacking, gunfire, an ambushed police officer and bombs thrown earlier. In spite of these extraordinary factors, they were able to capture this individual alive with no further harm to law enforcement. It was a tremendously effective outcome under dire circumstances.”