Police immediately began to search for the vehicle and pursued it into Watertown. During the chase, “explosive devices were reportedly thrown from car by the suspects,” the statement said, and the suspects and police exchanged gunfire in the area of Dexter and Laurel streets.

The two shooters, he said, had a large, unwieldy bomb that he said looked “like a pressure cooker.”

“They lit it, still in the middle of the gunfire, and threw it. But it went 20 yards at most.” It exploded, he said, and Tamerlan Tsarnaev ran toward the gathered police officers. He was tackled, but it was not clear if he was shot, Mr. Kitzenberg said.

The explosions, said another resident, Loretta Kehayias, 65, “lit up the whole house. I screamed. I’ve never seen anything like this, never, never, never.”

Meanwhile, Dzhokhar A. Tsarnaev, said Mr. Kitzenberg, got back into the SUV, turned it toward officers and “put the pedal to the metal.” The car “went right through the cops, broke right through and continued west.”

The two men left “a few backpacks right by the car, and there is a bomb robot out there now.” Police had told residents to stay away from their windows, he said.

During this exchange, an MBTA police officer was seriously injured and taken to the hospital.

At the same time, Tamerlan Tsarnaev was critically injured with multiple gunshot wounds and taken to Beth Israel Deaconess Hospital in Boston, where he was pronounced dead at 1:35 a.m.

A doctor who works at Beth Israel, and who lived in the area of the chase and shoot-out, said he was working at home around 1 a.m. when he heard the wailing sirens. He said at a news conference at Beth Israel that he recognized that something was wrong and alerted his emergency room to prepare for something.