Thousands of heavily armed police staged an intense manhunt Friday for a Chechen teenager suspected in the Boston marathon bombings with his brother, who was killed in a shootout.
One police officer was shot dead and another wounded in a violent cavalcade led by the two Muslim men, who had lived in the United States for years, that left the Boston region -- an area of roughly 900,000 people -- in a lockdown.
Dzhokhar Tsarnaev, 19, defied the massive force after his 26-year-old brother Tamerlan was shot and suffered critical injuries from explosives believed to have been strapped to his body.
The pair's social media pages appeared to express a sympathy with the struggles of Chechnya, which has been ravaged by two back-to-back wars since 1994 between Russia's army and increasingly Islamist-leaning separatist rebels.
A citywide alert issued after two bombs tore through crowds Monday at the Boston Marathon, killing three people and wounding about 180, reached a fever peak in the hours after the Federal Bureau of Investigation (FBI) released pictures of the two suspects on Thursday.
The brothers tried to rob a convenience store near the Massachusetts Institute of Technology (MIT) in Cambridge late Thursday, where a campus police officer was killed in a shootout, according to police.
A car driver was held up at gunpoint, but the driver was later freed and the two brothers took the car to Watertown, which was then 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 it was to prevent the fugitive from getting tipped off from television.
Streets were empty apart from police and armored vehicles brought into the town. Search helicopters patrolled the skies and sirens blared across the city as bomb squads carried out house-to-house searches.
Residents in Boston and the surrounding suburbs were told to stay put as authorities halted public transport, closed schools and universities and ordered most 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 shooting.
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.
"We're Muslims, we're Chechens, we're ethnic Chechens," Ruslan Tsarni, who said he was the uncle of the suspects, told US media.
"Somebody radicalized them. It's not my brother."
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," he said.
The attack on the marathon, which sent a hail of nails and shrapnel into a crowd of thousands at the finish line, 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, who was briefed early Friday by his national security team on developments in Boston, has pledged that the "evil" bombers would be brought to justice.
"Yes, we will find you, and yes, you will face justice," Obama said Thursday 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 have horrific injuries and will require new operations, doctors said.