Police on Friday surrounded a house in the Boston suburbs where the prime suspect in the deadly marathon bombings was believed to be hiding in a boat in the backyard, reports said.
The dramatic development came just minutes after authorities said Dzhokhar Tsarnaev, the 19-year-old believed to have carried out Monday's attack at the Boston Marathon with his brother Tamerlan, had not yet been found.
The brothers, who are of Chechen origin, are said to have killed a police officer in one shootout overnight before Tamerlan, 26, was fatally wounded in a second gun battle early Friday that seriously wounded a second officer.
Police squads and armored personnel carriers raced to Watertown in the evening hours and police helicopters buzzed overhead following a burst of gunfire erupted soon after authorities had told reporters the manhunt had not moved forward.
The Boston Globe quoted a source as saying police had the teenage suspect "pinned down." CNN said police believed they had the suspect surrounded in a boat in a backyard, with a 40-gallon tank of fuel aboard.
Throughout the day Friday, when authorities ordered people across the Boston area to stay behind locked doors, thousands of police went door-to-door in Watertown hunting for Tsarnaev, who was considered armed and dangerous.
There were widespread fears that the teen -- who was an all-star wrestler in high school -- had explosives strapped to his body.
Public transport in the city and Amtrak trains between Boston and New York were halted, while schools and universities, such as Harvard and Massachusetts Institute of Technology, were closed along with businesses. Barely a bar or restaurant was open in the ghostly city center streets.
But by late afternoon, state police chief Timothy Alben said the suspect was still at large, and Massachusetts Governor Deval Patrick lifted the so-called "shelter-in-place" order that had left 900,000 people cowering in their homes.
"We do not have an apprehension of our suspect this afternoon. But we will," Alben told reporters.
"For the sake of everyone that was hurt or killed during the marathon, or those police officers that lost their life, or were seriously injured, we are committed to seeing a conclusion to this case."
Tsarnaev's family and friends had appealed for him to surrender to police, who chased him armed with shotguns and automatic rifles, with the backing of helicopters and armored personnel carriers.
The brothers are Muslims who moved to the United States about a decade ago. Their social media pages appeared to express sympathy with the struggle of Chechnya, which has been ravaged by two wars since 1994 between Russia and increasingly Islamist-leaning separatist rebels.
A citywide alert issued after two bombs tore through crowds at the Boston Marathon, killing three people and wounding about 180, first reached a fever pitch late Thursday shortly after the Federal Bureau of Investigation released pictures of the two suspects.
A police officer was killed in a campus shootout with the suspects at the start of a violent cavalcade through the Boston suburbs, according to authorities.
The suspects then carjacked a car, setting off a police chase to Watertown. The two hurled explosives out of the window before the elder brother was shot. He died of bullet wounds and injuries from explosives strapped to his body, a doctor said.
A police officer was wounded in the clash and Tamerlan's use of explosives sparked the fears that his brother also had bombs on his body.
"We believe this to be a terrorist, we believe this to be a man who has come here to kill people," Boston Police Commissioner Ed Davis said of the younger Tsarnaev.
The suspects' father Anzor Tsarnaev told Russia's Interfax news agency his sons had been "set up by the secret services because they are practicing Muslims."
An uncle, Ruslan Tsarni, said his nephews "put a shame on the entire Chechen ethnicity" and that the teenaged Dzhokhar should give himself up and seek "forgiveness."
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.
President Barack Obama, who was briefed several times 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.
On Friday, he spoke to President Vladimir Putin and thanked him for Russia's anti-terror help after the bomb attacks in Boston.
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.