The sound of gunfire was reported Friday evening in Watertown, where authorities had been searching for the lone surviving suspect in the Boston Marathon bombings.
Emergency and military vehicles sped through town. Police told The Associated Press that multiple shots had been fired. Boston police said people should stay inside around a street in Watertown, 10 miles west of Boston.
It wasn't immediately clear whether authorities had found 19-year-old college student Dzhokhar Tsarnaev.
Tsarnaev had fled on foot after a furious overnight gun battle that left 200 spent rounds behind and after a wild car chase in which he and his brother hurled explosives at police, authorities said. His brother, 26-year-old Tamerlan Tsarnaev, died in the shootout, run over by his younger brother in a car as he lay wounded, according to investigators.
During the overnight spasm of violence, the brothers also shot and killed an MIT policeman and severely wounded another officer, authorities said.
Law enforcement officials and family members identified the brothers as ethnic Chechens who came to the U.S. from Russia. They lived near Boston and had been in the U.S. for about a decade, an uncle said.
Around midday, as the manhunt dragged on, the suspects' uncle Ruslan Tsarni of Montgomery Village, Md., pleaded on television: "Dzhokhar, if you are alive, turn yourself in and ask for forgiveness."
The search by thousands of law enforcement officers all but paralyzed the Boston area for much of the day. Officials shut down all mass transit, including Amtrak trains to New York, advised businesses not to open and warned close to 1 million people in the entire city and some of its suburbs to stay inside and unlock their doors only for uniformed police.
"We believe this man to be a terrorist," Boston Police Commissioner Ed Davis said. "We believe this to be a man who's come here to kill people."
Some neighborhoods resembled a military encampment, with officers patrolling with guns drawn and aimed, residents peering nervously from windows and people near surrounded buildings spirited away.
The bloody turn in the case came just hours after the FBI released photos and video of two suspects in the bombing and asked for the public's help in identifying and catching them.
Authorities said the man dubbed Suspect No. 1 -- the one in sunglasses and a dark baseball cap in the surveillance-camera pictures -- was Tamerlan Tsarnaev, while Suspect No. 2, the one in a white baseball cap worn backward, was his brother.
The bombings on Monday near the Boston Marathon finish line killed three people and wounded more than 180, tearing off limbs in a spray of shrapnel and sparking fears across the nation that another terrorist attack had come to U.S. soil.
Chechnya has been the scene of two wars between Russian forces and separatists since 1994, in which tens of thousands were killed in heavy Russian bombing. That spawned an Islamic insurgency that has carried out deadly bombings in Russia and the region, although not in the West.