Watertown

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Authorities say one of two suspects in the Boston Marathon bombing is dead and a massive manhunt is underway for another.

Residents of the Boston suburb of Watertown have been advised to keep their doors locked and not let anyone in.

The Middlesex district attorney says the two men are suspected of killing an MIT police officer at the college late Thursday, then stealing a car at gunpoint and later releasing its driver unharmed. Hours earlier, police had released photos of the bombing suspects and asked for the public's help finding them.