US police are carrying out a massive manhunt for a suspect in the Boston Marathon bombing after a second suspect was killed in a shootout with officers.
Police are locking down some neighbourhoods in Boston and its western suburbs as they search for the remaining suspect known as the man in the white hat from marathon surveillance footage.
Authorities urged residents in Watertown, Cambridge and other towns west of Boston, as well as the Allston-Brighton neighbourhoods of western Boston, to stay indoors. All public transport was shut down and businesses were asked not to open. People waiting at bus and subway stops were told to go home.
"We believe this to be a terrorist," said Boston Police Commissioner Ed Davis of the suspect on the loose. "We believe this to be a man who's come here to kill people. We need to get him in custody."
The Middlesex district attorney said the two men are suspected of killing a Massachusetts Institute of Technology police officer on campus in Cambridge in a late night shooting, then stealing a car at gunpoint and later releasing its driver unharmed.
Hours earlier, police had released photos of the marathon bombing suspects and asked for the public's help finding them. A new photo of the suspect on the loose was released later showing him in a grey hoodie sweatshirt. It was taken at a 7-Eleven store in Cambridge, just across the Charles River from Boston.
The first images were released hours after President Barack Obama and first lady Michelle Obama attended an interfaith service at a Roman Catholic cathedral in Boston to remember the bombing victims.
Authorities say the suspects threw explosives from the car as police followed it into Watertown. The suspects and police exchanged gunfire, and one of the wanted men was critically injured and later died at a hospital while the other escaped. Doctors at Beth Israel Deaconess Medical Centre in Boston where the suspect was taken said they treated a man with a possible blast injury and multiple gunshot wounds. They wouldn't say if he was the suspect in the black hat.
The MIT shooting on the Cambridge campus was followed by reports of gunfire and explosions in Watertown, about 10 miles west of Boston. The MIT officer had been responding to report of a disturbance when he was shot multiple times, according to authorities. There were no other victims.
In Watertown, witnesses reported hearing multiple gunshots and explosions at about 1 am local time. Dozens of police officers and FBI agents were in the neighbourhood and a helicopter circled overhead.