Images taken from a video released by the FBI
Police were searching for the man in the Boston suburb of Watertown. They said they were looking for the bombing suspect who was photographed wearing a white hat just before the Monday explosions that killed three people and wounded 176.
Police on Friday killed one suspect in the Boston Marathon bombing during a shootout and mounted a house-to-house search for a second man after a bloody night of shooting and explosions in the city's streets.
A national security official identified the hunted man as Dzhokar A Tsarnaev (19) and said the dead suspect was his brother, Tamerlan Tsarneav (26).
The brothers had been in the United States for several years, the official said.
Authorities locked down much of Watertown and told residents not to leave their homes and not to answer the door as they scoured the area for the missing man, who was described as armed and dangerous. Public transportation throughout the Boston metropolitan area was suspended, and air space was restricted.
During the night a university police officer was killed, a transit police officer was wounded, and the suspects carjacked a vehicle before leading police on a chase that ended with one suspect shot dead.
Police were searching for the man known as suspect two, who was shown wearing a white cap in surveillance pictures taken shortly before Monday's explosions and released by the FBI on Thursday.
Suspect one, the man wearing a dark cap and sunglasses in the same images, was dead, officials said. "We believe this to be a terrorist," said Boston police commissioner Ed Davis of the suspect still at large. "We believe this to be a man who has come here to kill people. We need to get him in custody."
Searches
Wave after wave of heavily armed police with tactical gear, long rifles and full armor descended on Watertown conducting house-to-house searches inside a 20-block area.
Twin blasts believed caused by bombs in pressure cookers placed inside backpacks left near the finish line of Monday's Boston Marathon killed three people and wounded 176 in the worst attack on US soil since the suicide hijacking attacks of September 11 2001.
Police said the two men had explosives with them during their skirmishes with officers on Thursday night and into Friday morning, requiring a number of controlled explosions to destroy suspected unexploded ordnance.
As police closed in on the two men overnight they were attacked with explosives and gunfire before one suspect was shot and taken to a hospital, where he died.
Step by step
About five hours after the surveillance pictures were released and shown widely on television and on the internet appeared on later, a university police officer was shot and killed on the campus of the Massachusetts Institute of Technology, the Middlesex county district attorney said in a statement.
A short time later, police received reports of a carjacking by two men who kept their victim inside the car for about half an hour before releasing him, the statement said.
Police pursued that car to Watertown, where explosives were thrown from the car at police and shots were exchanged, the statement said. "During the exchange of the gunfire, we believe that one of the suspects was struck and ultimately taken into custody. A second suspect was able to flee from that car and there is an active search going on at this point in time," Colonel Timothy Alben, superintendent of the Massachusetts state police, told a news conference.
The wounded suspect was taken to Beth Israel Deaconess Medical Centre, where he died, said Dr Richard Wolfe, chief of emergency medicine.
"This was a trauma arrest, multiple injuries, probably, we believe, a combination of blast, potentially gunshot wounds," Wolfe told a news conference.
When asked how many gunshot wounds, he said: "Unable to count."
The blast injuries may have been caused by "an explosive device, possibly shrapnel, thermal injury. It was pretty much throughout the trunk. It was multiple wounds," he said.
Transportation shut down
Massachusetts Governor Deval Patrick suspended all public transportation service on the Boston-area subway, bus and rail system as a precaution, said Kurt Schwartz, director of the Massachusetts emergency management agency.
Schwartz also asked people in the Boston-area communities of Newton, Waltham, Belmont, Cambridge and the Allston-Brighton to stay indoors and asked businesses in those areas to remain closed pending further notice.
The Massachusetts Institute of Technology said it canceled all classes on Friday after one of its police officers was killed. The US Federal Aviation Administration closed low-level airspace over Boston.
US President Barack Obama was briefed overnight by a counterterrorism aide on the Boston bombing investigation and manhunt, a White House official said. Neighbours reported hearing several gunshots and explosions in Watertown, a suburb to the Massachusetts Institute of Technology where the campus police officer was killed.
Rebecca Carbone (30) stood on the street wearing pajamas and a sweatshirt and had stepped out when she heard sirens. "We heard a loud blast and we didn't know what it was," Carbone said. "It sounded like a car backfiring."
John Grimes (69), a retired letter carrier, said that he heard three loud explosions, "and you don't hear explosions at night a lot." – Reuters