Two young men, armed with guns and explosives in what appeared to be backpacks, engaged in a violent standoff with dozens of police on a street in Watertown, Mass., late Thursday night, a resident said.

Police officers blocked off the scene of a shooting at the Massachusetts Institute of Technology in Cambridge, Mass.

Andrew Kitzenburg, 29, said he looked out of his third floor window to see two young men of slight build in jackets shooting at dozens of police officers from behind a black Mercedes SUV. The officers and the men were 70 yards apart, he said, and engaged in “constant gunfire.”

A police SUV “drove toward the shooters,” he said, and was shot at until it was severely damaged. It rolled out of control, Mr. Kitzenberg said, and crashed into two cars in his driveway.

The two shooters, he said, had a large and unwieldy bomb. “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 one of the two men ran towards the gathered police officers. He was tackled, but it was not clear if he was shot, Mr. Kitzenberg said.

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

Meanwhile, the other you man, 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. The standoff came days after two bombs had exploded at the Boston marathon, killing three and injuring more than 170. There was speculation that the two incidents were linked but, amid the chaos early Friday morning, there was no confirmation of that. The Federal Bureau of Investigation released new images of the men early Friday as part of a campaign to try and identify them.

The incident in Watertown came just minutes after a third explosion of violence in Boston — a campus police officer was shot and killed at the Massachusetts Institute of Technology.

The officer, who was not named, responded to a report of a disturbance near Vassar and Main Streets, the Middlesex County District Attorney Michael Pelgro said in a statement early Friday. He was found, the statement said with “multiple gunshot wounds” and taken to Massachusetts General Hospital where he was pronounced dead.

At the campus, helicopters whirred overhead, and police cars were dotted through the streets. A crime scene was cordoned off, and at least one dog unit was on the scene.

NY Times