On Thursday evening, authorities released photos of the two men, who had been spotted carrying backpacks near the marathon’s finish line.
Their targets, it turned out, had not fled the city or the country. A few hours later, they began a violent rampage just across the Charles River in Cambridge.
About 10:30 p.m., authorities said, the two shot and killed MIT police officer Sean Collier, 26, as Collier sat in his cruiser. It was unclear what triggered that shooting: authorities said surveillance video appears to show the two approaching Collier and killing him without warning.
After that, the men allegedly carjacked a Mercedes-Benz sport utility vehicle, and took the driver with them. At least one brother told the carjacked driver that they were the marathon bombers, officials said. They forced the driver to stop at several bank machines, and took $800 that he withdrew. After a few minutes, the man was left behind at a gas station, unharmed.
From there, the brothers drove about three miles. In Watertown, they engaged in a shootout with police. At some point, the men threw homemade explosives at officers. Boston Transit police officer Richard H. Donahue, 33, was wounded.
During that shootout, authorities said that Tamerlan Tsarnaev left the car, and police tackled him in the street. Then, authorities said, Dzhokhar Tsarnaev — still behind the wheel of the car — swerved at the officers in an effort to hit them.
The officers dodged. Tamerlan did not. He was dragged under the car, and later died at a Boston hospital.
Dzhokhar Tsarnaev escaped on foot, as arriving officers failed to set up a secure perimeter.
It was the first of two times Friday that he would evade a police manhunt. Later in the day — as state authorities issued a massive order for nearly 1 million people to “shelter in place” — officers conducted house-to-house searches over 20 blocks of Watertown.
Tsarnaev, it turned out, had fled to a house that was just outside that search zone. By 6 p.m., authorities conceded that they hadn’t found him and couldn’t be sure where he was.
They lifted the order to stay home. Just after that, the resident in Watertown walked outside and saw the blood. A police helicopter used infrared technology to spot movement underneath the plastic cover.
Inside, Tsarnaev had been wounded by the firefight hours earlier. He may have been wounded again by the exchange of gunfire with officers that surrounded the boat.
Officers tried to negotiate his surrender. There was no response. Finally, a robot pulled back the cover, and the SWAT team pulled him out. He was wounded in the leg and neck.
Clarence Williams, Ed O’Keefe and Jenna Johnson and researcher Julie Tate contributed to this report.
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