People across Boston and surrounding suburbs have been told to stay indoors amid a massive police manhunt for one of two men suspected of carrying out the Boston Marathon bombings.Dzhokhar Tsarnaev, 19, remains at large after he escaped a shoot-out in which another suspect, his brother, died.
"Stay indoors with the doors locked," the Massachusetts governor has said.
Three people died and more than 170 were hurt when two bombs exploded near the finish line of Monday's marathon. Terror in Boston
- Thursday 22:20 (03:20 BST, 19 April) Police officer shot dead at the Massachusetts Institute of Technology campus in Cambridge, near Boston
- Two men, later identified as the Boston marathon bomb suspects, hijack a car at gunpoint
- 01:00 (06:00 BST) Police chase the car as it heads to Watertown
- One suspect is fatally injured and the other flees after a shoot-out with police
- Police call for lockdown of Watertown and surrounding area as the manhunt continues
The FBI released several images of two men they were hunting in relation to the bombing, one wearing a white cap, the other a black cap.Police said "suspect number one" had been killed early on Friday, and they were looking for "suspect number two", the "white-capped individual", later named as Dzhokhar Tsarnaev.
The black-capped suspect - widely named in the US media as Tamerlan Tsarnaev - was the fugitive's elder brother.
Both are said to be of Chechen origin, and are reported to have moved to the United States about 10 years ago.
Blast wounds The manhunt began late on Thursday when university police officer Sean Collier, 26, was killed on the Massachusetts Institute of Technology (MIT) campus.
Two men carjacked a driver at gunpoint and drove away with the driver still in the car. They later released the man unharmed.
Police chased the suspects, who threw bombs and exchanged gunfire with police, seriously wounding one officer.
In the Boston suburb of Watertown, officers and the men were involved in a gun battle lasting 10 minutes, according to witnesses.
They'd gone out late at night to pick up their new pet and when they came back were told by police they couldn't return to their home in Watertown - there was a terrorist on the loose, with explosives.
We and they are on one side of the cordon. Inside those 20 blocks it must be terrifying - to be told to stay inside, behind a locked door. This is now the advice for the whole of Boston. The police obviously don't want to create panic but they would rather people were frightened than hurt.
America wasn't cowed by the bombings, but having one of its oldest cities shut down, in fear, may have a greater impact.
The authorities in Massachusetts Bay have suspended the transport system and no vehicles are being allowed in or out of the Watertown area.The warning to stay indoors was later extended to the whole of Boston, in what correspondents said was an unprecedented move.
"Stay indoors with the doors locked, and do not open the door for anyone other than a properly identified law enforcement officer," Massachusetts Governor Deval Patrick said on Friday morning.
Gunshot wounds Meanwhile, President Barack Obama was briefed on developments in the manhunt and investigation for about an hour in the White House Situation Room.
US Secretary of State John Kerry, also at the briefing on video link, said the authorities were "part of the way there" in bringing the Boston terror suspects to justice, AP reported.
Overnight, 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 the Watertown area was a "terrorist".
"We believe this to be a man who came here to kill people," he said.
A grey Honda CRV vehicle, which reports said had been sought in connection with the suspects, was found on Friday morning in the Boston area, Connecticut police said.
The father of Dzhokhar Tsarnaev said his son was a second year medical student in the US and was hoping to be a brain surgeon.
But Anzor Tsarnaev told the BBC that that he believed the secret services had framed his sons.
Monday's attack on the Boston Marathon killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, 23, a postgraduate student from China.