The FBI has released several images of the suspectsUS police are hunting a suspect in Monday's deadly Boston Marathon bombing, after another suspect was killed in a car chase and shoot-out.
Police began the manhunt on Thursday after an officer was killed at a university campus.
Two suspects stole a car and threw bombs and traded gunfire with police chasing them. An officer was wounded.
Three people were killed and more than 170 wounded when two bombs exploded near the finish line of the marathon.
The authorities tracked the two suspects to Watertown, west of Boston, where one was killed and another escaped.
Police later confirmed that the two men they were chasing were suspects in Monday's marathon bombing.
The FBI has released several images of two men they were hunting in relation to the bombing.
One was wearing a white cap, the other had a black cap.
Police said "suspect number one" had been killed, and they were looking for "suspect number two", the "white-capped individual".
Col Timothy Alben: Suspect is "armed and dangerous"
The authorities in Massachusetts Bay have suspended the transport system and no vehicles are being allowed in or out of the Watertown area.
Officials have told people in the area to stay at home, and warned businesses not to open.
Armed police Swat teams are going from door-to-door in Watertown searching for the suspect.
An earlier police statement said the two men were suspected of killing a police officer on campus at the Massachusetts Institute of Technology (MIT) late on Thursday.
They then stole a car at gunpoint and drove away with the car's owner still on board.
They released the driver unharmed.
After a car chase, witnesses in Watertown heard several explosions and a gun-battle lasting 10 minutes.
Video footage emerged showing a fully-clothed suspect lying on the floor, surrounded by police.
More video was shown by US media of a suspect being led into a police car after being stripped of his clothes.
But it is not clear who the men were - and what happened after their apparent arrest.
Dr Richard Wolfe, of the Beth Israel Deaconess Medical Center, said the suspect was brought in with multiple blast and gunshot wounds to his upper body.
He was in cardiac arrest when he arrived at hospital and despite attempts to resuscitate him, he was pronounced dead at 01:35 (05:35 GMT), Dr Wolfe said. .
Police also confirmed that an officer had been critically wounded in the chase.
Boston police Commissioner Ed Davis said he believed the man being hunted in Watertown was a "terrorist".
"We believe this to be a man who came here to kill people," he said.
Three people were killed and more than 170 wounded when the two bombs exploded near the finish line of the Boston Marathon.
The blasts killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, 23, a postgraduate student from China.