Pictures of the Boston Marathon bombing suspects have emerged for the first time, alongside dramatic images of a shoot-out and explosions in Watertown and at MIT, where a police officer was killed.
One suspect is dead and another still at large.
A police spokesman described the man they were looking as a "terrorist", of Caucasian descent with curly brown hair, adding that he was armed and dangerous. Police have warned residents to stay in their homes.
Officials said the streets were now in lockdown as they searched for the man who was first pictured in CCTV from the Boston Marathon wearing a white hat. They refused to name either suspect.