Large parts of the city of Boston remain in virtual lockdown amid a major manhunt
for one of two brothers suspected of bombing the city's marathon on Monday.
Dzhokhar Tsarnaev, 19, remains at large after he escaped a shoot-out in which another suspect, his brother, died.
Police said they had searched 60-70% of a locked-down area of a Boston suburb.
Three people died and more than 170 were hurt when two bombs exploded near the finish line of Monday's marathon.
On Friday afternoon, Col Timothy Alben of the Massachusetts State Police said officers in Watertown were searching "door to door, street to street" for the suspect, but there was as yet no word on his whereabouts.
"Things change, they change quickly," the officer said, adding: "We are working on several new leads that have just developed in the last few minutes."
Massachusetts Governor Deval Patrick said there were "continuing developments" in the investigation, and that an order to stay indoors remained in place across the whole of Boston and surrounding suburbs.
Earlier the FBI released images of the two men they were hunting in relation to the bombing.
Police said one suspect - widely named in the US media as Tamerlan Tsarnaev - had been killed early on Friday, and they were looking for another suspect, later named as the dead man's younger brother Dzhokhar Tsarnaev.
Both are said to be of Chechen origin, and are reported to have moved to the United States about 10 years ago.
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 vehicle at gunpoint 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.
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.
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.
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.
The authorities were investigating whether the dead man had a home-made bomb strapped to his body when he was killed, reports 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 he believed the secret services had framed his sons.
"It was a terrorist attack carefully organised by secret services - I don't know which ones. My son used to go to a mosque, so they once paid us a visit to ask why he is doing that.
"Yes, there was such an episode. So they put all the blame on him and shot him. That's it."
Ruslan Tsarni, an uncle of the suspects who lives in Maryland, said he was "ashamed" of their alleged involvement in the bombings.
He urged Dzhokhar Tsarnaev to "turn himself in".
"Yes of course we're ashamed, they're the children of my brother," he said.
Asked what the bombers' motives may have been, he replied: "Being losers, hating everyone around them."
Monday's attack on the Boston Marathon killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, 23, a postgraduate student from China.