The shooting in Watertown has been linked to the Boston bombing, according to law enforcement officials. One of the suspects who was shot dead in Watertown was a Boston bombing suspect, police say
Police are lining on Arsenal Street in and out of cars for more than a 2 mile (3.3km) stretch, according to local reporters at the scene.
One of the suspects may be inside a building in Watertown, according to a police officer quoted by NBC.
A transit police officer is being treated for a gunshot wound, while one of the suspects has been taken to hospital.
Officer down.