Man who witnessed shooting
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Witnesses hear explosions and gunfire in Watertown

Witnesses have described hearing explosions and gunfire in a town outside Boston following the fatal shooting of a police officer at the Massachusetts Institute of Technology (MIT) in Cambridge.

One man in Watertown described seeing, "unmarked cars, marked police cars... I could tell they were serious because he drew a pistol at me and said 'Go back, go home'".

The operation has been linked to the Boston Marathon bombings.