US 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 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.
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 of a suspect lying on the floor, and later being led into a police car after being stripped of his clothes.
Officials later said one suspect had been hit by bullets during the chase and died.
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.
Watertown resident describes explosions and gunfire
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.
It was unclear whether the arrested man on the video footage was the suspect who had died.
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.
WATERTOWN, Mass. -- One of the suspected Boston Marathon bombers was killed in a shootout with police, and the other remains on the loose Friday morning, after the pair went on a rampage that left one police officer dead and another critically wounded, authorities said.
As the manhunt continued for suspect No. 2 -- the one pictured with a white cap at the marathon Monday -- Gov. Deval Patrick has ordered the immediate shutdown of public buses and trains in metropolitan Boston as a precaution.
"We believe this to be a terrorist," Boston police commissioner Ed Davis said. "We believe this to be a man who's come here to kill people. We need to get him into custody."
Authorities urged residents living in Watertown, Newton, Waltham, Belmont, Cambridge and Allston-Brighton to remain in their homes. They also asked businesses not to open.
Both unidentified suspects were involved in the fatal shooting of a Massachusetts Institute of Technology police officer before carjacking a Mercedes SUV and driving it into Watertown, where police pursued and confronted them in a residential neighborhood, said Col. Timothy Alben of the Massachusetts State Police.
"We believe these are the same individuals that were responsible for the bombing on Monday at the Boston Marathon," Alben said.
During the confrontation in Watertown, "there was an exchange of gunfire" and "several explosive devices" were directed at the officers, some of them detonated and some did not, Alben said. At that point one of the suspects was struck and taken into custody, while the other fled. The Middlesex County district attorney's office reported that the suspect in custody had died.
Doctors who treated the dead bomber at Beth Israel Hospital said he suffered multiple gunshot wounds as well as injuries consistent with an explosive blast, possibly involving shrapnel.
A source said authorities believe there are undetonated improvised explosive devices in Watertown tossed there by the suspects.
"They were throwing bombs out of the car as police pursued them," the source said. "They were looking to inflict maximum damage and death."
Davis said said there were some unexploded devices that the men had hurled, and bomb squads were trying to disarm them.
Witness Quentin Copper said the suspects were "fearless."
"They drove right at the cops and threw bombs out the windows at them, like a video game," Cooper said.
The drama began Thursday night near the campus of MIT, in Cambridge, where authorities said the men robbed a convenience store near campus before fatally shooting a male MIT police officer, according to the Middlesex district attorney's office. The officer was later pronounced dead at Massachusetts General Hospital.
The suspects then carjacked the Mercedes SUV, officials said. Authorities encountered the SUV and a high-speed chase ensued through the usually quiet Cambridge streets and into the commercial district of Watertown about four miles away.
State police said the carjacked Mercedes stopped and the suspects opened fire on the pursuing police officers. A fierce firefight ensued, and one of the suspects lit what officials described as an explosive and threw it in the officers' direction, police said.
A second police officer was shot during the shootout and was in critical condition.
AP sources: Boston bomb suspects from Russia region near Chechnya, lived in US at least 1 year
Islam is the predominant religion in Chechnya. Chechens are overwhelmingly adherents to Sunni Islam, the country having converted to Islam between the 16th and the 19th centuries. Due to historical importance, many Chechens are Sufis, of either the Qadiri or Naqshbandi orders. Most of the population follows either the Shafi'i, Hanafi, or Maliki schools of jurisprudence, fiqh. The Shafi'i school of jurisprudence has a long tradition among the Chechens, and thus it remains the most practiced.