BOSTON -- Blocks from where the marathon bombs had exploded three days earlier, FBI agents stood beside easels Thursday afternoon in a ballroom of the Sheraton Boston Hotel. On cue, they unveiled dramatic surveillance photos of the brothers suspected of the terror attack.
Those images had been public less than five hours when, across the Charles River in Cambridge, the brothers' time on the run took a deadly turn.
Officer Sean Collier of the Massachusetts Institute of Technology's campus police force responded about 10:20 p.m. to a disturbance call at the corner of Main and Vassar streets.
Gunshots rang out. Collier was hit as he sat in his patrol car.
The men involved in the shooting, investigators would quickly realize, weren't ordinary criminals. They were the most wanted men in America.
Gunfight in suburb
It was the latest spasm of terror that would hit the region. There would later be a huge firefight in the tranquil suburb of Watertown, where police would catch up with the suspects, and more than a million people in the Boston area would become virtual prisoners in their own homes as police searched door to door.
The MIT shooting was the start of a chaotic, terrifying night.
Leaving the mortally wounded officer behind, Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev, 19, fled in a Honda CR-V, authorities said.
A short time later, police said, the pair carjacked a Mercedes SUV.
"We did the bombings. We just killed a cop. And if you don't give us the car, we'll kill you, too," one of the brothers told the driver of the Mercedes, according to a federal law enforcement source.
The brothers held the man captive and used his cash card to withdraw a total of $800 from several ATMs, the source said. For unknown reasons, they decided to let the unharmed driver go free.
Cops investigating the carjacking contacted Mercedes, hoping the car company could help pinpoint the vehicle using GPS. Before the company could act, Watertown police spotted the fleeing vehicle, said Dave Procopio, a spokesman for the Massachusetts State Police.
The suspects tossed pipe bombs and a grenade out of the vehicle at pursuing cops, police said.
While details of what happened next aren't clear, just after 1 a.m. yesterday a furious gunfight between cops and the brothers shook the Armor Street area of Watertown awake. Gunshots were followed by explosions, then more gunshots and explosions.
"There are live explosives on the ground!" one officer screamed. "Get out of here!"
Another officer hurt
Guns brandished, cops sprinted through the darkened streets, taking cover behind parked cars. A transit police officer, 33-year-old Richard Donohue Jr., was shot while exchanging fire with the suspects, authorities said. He was in surgery yesterday and expected to recover.
Tamerlan Tsarnaev was fatally shot by police, authorities said. In his haste to get away, Dzhokhar Tsarnaev drove the Mercedes over his dying brother's body, The Boston Globe reported, citing two anonymous sources.
"They drove right at the cops and threw bombs out the windows at them, like a video game," said resident Quentin Cooper, who witnessed the drama.
Cooper said he saw the Mercedes crash through a police barricade to escape.
"It sounded like the end of the world," said another Watertown resident, John Timmons.
At least 200 rounds were exchanged in the firefight, police said, and Tsarnaev later abandoned the stolen vehicle and fled on foot.
Throughout the day, police in body armor went house to house, searching for the remaining fugitive. Massachusetts Gov. Deval Patrick ordered the city of Boston and its surrounding suburbs locked down and its residents to remain in their homes for much of yesterday.
With Chau Lam