This is what the BBC has on the situation at MIT.

A gunman has shot dead a police officer at the Massachusetts Institute of Technology (MIT) in Cambridge near Boston, officials say.

Cambridge police say the officer was responding to a report of a disturbance in the area when he was shot and suffered "multiple wounds". He was taken to hospital where he was pronounced dead.

Witnesses in nearby Watertown report hearing explosions and gunfire amid a massive police hunt for the gunman.Television footage from the town, some six miles (9km) from Boston, showed one man lying spread out on the ground, holding out his arms, surrounded by police.

There has been no official comment on the police operation, and whether it is linked to the shooting at the MIT campus or the bombings at the Boston Marathon on Monday.

The MIT campus is currently on lockdown.A statement on the university's website says a building there is surrounded by the emergency services.

The statement says the situation is considered "extremely dangerous".TV footage showed police swarming around the university's Stata Center.The university is advising students to stay clear of the area until further notice.A statement from the Middlesex district attorney's office said there are no other victims.