Major police operation in Boston area after shooting
A major police operation is under way in a town outside Boston following the fatal shooting of a police officer at the Massachusetts Institute of Technology (MIT) in Cambridge.
Large numbers of police descended on Watertown. Witnesses reported hearing explosions and gunfire.
TV footage showed a man being led naked to a police car. A second suspect is believed to be on the run.
It is not clear if there is any link to Monday's Boston Marathon bombings.
The FBI, which is leading the hunt for those behind the Marathon attacks, is reported by US media to be on the scene at Watertown.
Cambridge police said the officer was responding to a report of a disturbance in the area of the MIT campus at around 22:30 (02:30 GMT) when he was shot and suffered "multiple wounds".
He was taken to hospital where he was pronounced dead.
Following reports of a carjacking in the area, a major police operation then unfolded in Watertown, some six miles (10km) from Boston.
Reporters arriving at the scene were told to turn off their mobile phones. One police officer reportedly told them: "If you want to live, turn off your cell phone."
Many witnesses described hearing booms and gunfire. One resident, 30-year-old Rebecca Carbone, said she head a loud blast that "sounded like a car backfiring".
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."
Television footage showed one man lying spread out on the ground, holding out his arms, surrounded by police. Later footage showed a naked man being led away by police.
There are reports that a second suspect is being hunted by police, but the details remain sketchy.
Police have said they are going door-to-door in Watertown due to what they describe as a "fluid situation", warning residents not to open their doors to anyone other than a clearly identified police officer.
There has been no official word on whether the operation in Watertown is linked to either the shooting at the MIT campus or the bombings at the Boston Marathon. The police tweeted earlier: "There is an active incident ongoing in Watertown. Residents in that area are advised to remain in their homes".
The campus of the prestigious MIT was on lockdown following the shooting, and police could be seen swarming around the university's Stata Center.
The university later issued a statement to say the campus is now safe but urged students to "remain vigilant".
Earlier, the FBI released photos of two suspects it said it wanted to identify as part of its investigation into the bombings at the Boston Marathon that killed three people and wounded more than 170.