Thousands of heavily armed police staged a house-to-house hunt Friday for a teenager suspected of carrying out the Boston marathon bombings with his brother, who was killed in a wild shootout.
One police officer was shot dead and another wounded in a violent cavalcade led by the two men, Muslims believed to be of Chechen origin, that left much of the Boston region -- an area with roughly 900,000 people -- in a lockdown.
Dzhokhar Tsarnaev, 19, defied the massive hunt force after his 26-year-old brother Tamerlan was shot and suffered critical injuries from explosives believed to have been tied to his body.
The surviving fugitive was also feared to be strapped with explosives.
An alert since two bombs tore through crowds at the Boston Marathon on Monday -- killing three people and wounding about 180 -- reached a feverish peak in the hours after the Federal Bureau of Investigation (FBI) released pictures of the two on Thursday.
The brothers, legal residents of the United States, tried to rob a convenience store near the Massachusetts Institute of Technology (MIT) in Cambridge late Thursday. They then went to the prestigious university where a campus police officer was killed in a first shootout, according to police.
A car driver was held up at gunpoint, but the driver was freed and they took the car to Watertown, which was sealed off by more than 9,000 police armed with shotguns, assault rifles and other weapons.
Police forced television stations to broadcast their coverage with a delay, saying this was to stop the fugitive getting details of the hunt from television.
Streets were empty apart from police and armored vehicles brought into the town. A police helicopter hovered over Norfolk Street in Watertown, where the brothers had a house.
Residents in the greater Boston area were told to stay home as authorities halted public transport, closed schools and universities and ordered many businesses shut.
"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 told reporters.
Boston Police Commissioner Ed Davis described Dzhokhar Tsarnaev -- a baby-faced teen who was an all-star wrestler in high school -- as "armed and dangerous."
"We believe this to be a terrorist, we believe this to be a man who has come here to kill people."
Tamerlan Tsarnaev died after suffering multiple gunshot wounds and an injury in an explosion, a doctor at Beth Israel hospital, David Schoenfeld, told reporters. Davis said one police officer was seriously wounded in the gun battle.
During a car chase through Watertown before the final shootout, the brothers were seen throwing explosives out of the car, local media said, citing police reports. Blasts and gunfire were heard in several districts.
Police with rifles flooded the streets of Boston, and search helicopters patrolled the skies. Sirens blared across the city as bomb squads carried out house-to-house searches.
A man who identified himself as the father of the pair, Anzor Tsarnaev, told Interfax news agency his sons were innocent: "In my opinion, my children were set up by the secret services because they are practicing Muslims."
The attack on the marathon, which sent a hail of nails and shrapnel into a crowd of thousands at the end of the marathon, was the worst terror assault on the United States since the September 11, 2001 suicide airliner attacks.
Just hours before the chaotic manhunt, the FBI on Thursday released pictures and video of the two suspects, appealing for help to identify the pair, who were carrying large backpacks.
One -- now identified as Dzhokhar -- was wearing a white baseball cap and the other -- Tamerlan -- a black cap.
The men are seen in the video walking calmly, one a few paces behind the other, weaving between crowds on Boston's Boylston Street where the race finished.
President Barack Obama vowed to the people of Boston Thursday that the "evil" bombers would be brought to justice.
"Yes, we will find you, and yes, you will face justice," Obama said at a service at Boston's Cathedral of the Holy Cross attended by 2,000 people including blast survivors, relatives of the dead, rescuers and city leaders.
"If they sought to intimidate us, to terrorize us," Obama said, "it should be pretty clear by now that they picked the wrong city to do it."
Boston has held emotional tributes to the dead -- eight-year-old Martin Richard, Boston University graduate student Lu Lingzi of China and Krystle Campbell, a restaurant manager. Obama paid tribute to all three at the service.
More than 100 of the wounded have left Boston hospitals and fewer than 10 of those still in hospital remain in critical condition. some with horrific injuries. Some will require new operations, doctors said.