One of two suspects in the deadly Boston Marathon bombing is dead and a massive manhunt is underway for another, authorities said early Friday.
Residents of Watertown, a suburb just outside Boston, have been advised by police to keep their doors locked and not let anyone in.
"We believe this to be a terrorist," said Boston Police Commissioner Ed David. "We believe this to be a man who's come here to kill people. We need to get him in custody."
The Middlesex district attorney said the two men are suspected of killing a Massachusetts Institute of Technology police officer on campus late Thursday, then stealing a car at gunpoint and later releasing its driver unharmed.
Hours earlier, police had released photos of the marathon bombing suspects and asked for the public's help finding them.
A new photo of the suspect on the loose was released later showing him in a grey-hooded sweatshirt. It was taken at a 7-Eleven store in Cambridge, just across the river from Boston.
Authorities say the suspects threw explosives from the car as police followed it into Watertown. The suspects and police exchanged gunfire, and one of the suspects was critically injured and later died at a hospital while the other escaped.
The FBI said it was working with local authorities to determine what happened.
The MIT shooting on the Cambridge campus Thursday night was followed by reports of gunfire and explosions in Watertown, about 10 miles (16 kilometers) west of Boston.
The MIT officer had been responding to report of a disturbance Thursday night when he was shot multiple times, according to a statement from the Middlesex district attorney's office and Cambridge police. It said there were no other victims.
In Watertown, witnesses reported hearing multiple gunshots and explosions at about 1 a.m. (0500 GMT) Friday. Dozens of police officers and FBI agents were in the neighborhood and a helicopter circled overhead.
State police spokesman David Procopio said, "The incident in Watertown did involve what we believe to be explosive devices possibly, potentially, being used against the police officers."
Boston cab driver Imran Saif said he was standing on a street corner at a police barricade across from a diner when he heard an explosion.
"I heard a loud boom and then a rapid succession of pop, pop, pop," he said. "It sounded like automatic weapons. And then I heard the second explosion."
He said he could smell something burning and advanced to check it out but area residents at their windows yelled at him, "Hey, it's gunfire! Don't go that way!"
MIT said right after the 10:30 p.m. (0230 GMT) shooting that police were sweeping the campus in Cambridge and urged people to remain indoors. They urged people to stay away from the Stata Center, a mixed-use building with faculty offices, classrooms and a common area.
Hours later, MIT, the prestigious university with about 11,000 students, said the campus was clear but the shooter was still on the loose.