Heavily armed police are searching a US town house by house for a man suspected of bombing the Boston Marathon.
Residents of Watertown and surrounding areas, including the whole of Boston, have been warned to stay indoors.
Police are hunting Dzhokhar Tsarnaev, said to be 19 and of Chechen origin, after he escaped a shootout with police in which a second suspect died.
Three people died and more than 170 were hurt when two bombs exploded near the finish line of Monday’s marathon.
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”, later named as Dzhokhar Tsarnaev.
Reports said the black-capped suspect was the fugitive suspect’s elder brother.
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 vicinity to stay at home, and warned businesses not to open.
The manhunt began late on Thursday when a police officer was killed on campus at the Massachusetts Institute of Technology (MIT).
Two men carjacked a driver at gunpoint and drove away with the driver still in the car. They released the man unharmed.
Police chased the suspects, who threw bombs and exchanged gunfire with police, seriously wounding one officer.
In Watertown, officers and the men were involved in a gun battle lasting 10 minutes, according to witnesses.
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, or what happened after their apparent arrests.
Dr Richard Wolfe, of the Beth Israel Deaconess Medical Center, said an individual 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.
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.
Monday’s attack on the Boston Marathon killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, 23, a postgraduate student from China.