In a possible break in the case, gunfire erupted Friday night amid the manhunt for the surviving suspect in the Boston Marathon bombing, and police in armored vehicles and tactical gear rushed into the Watertown neighborhood.

The burst of activity came at the end of a tense day in and around Boston, and less than an hour after police announced that they were scaling back the hunt because they had come up empty-handed following an all-day search that sent thousands of SWAT team officers into the streets and paralyzed the metropolitan area.

Less than an hour after the hail of gunfire, a round of blasts could be heard. A state police spokesman said only that the activity was related to the search for 19-year-old college student Dzhokhar Tsarnaev.

Boston Mayor Tom Menino told WBZ-TV the suspect was holed up in a boat parked in a backyard. Reporters were being kept away from the scene and a message left for Menino's spokeswoman wasn't immediately returned.

Before the gunfire, State Police Col. Timothy Alben said at a news conference that he believed Tsarnaev was still in Massachusetts because of his ties to the area. But authorities lifted the stay-indoors warning for people in the Boston area, and the transit system started running again by evening.

"We can't continue to lockdown an entire city or an entire state," Alben said. At the same time, he and other authorities warned that Tsarnaev is a killer and that people should be vigilant.

Tsarnaev 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.

But investigators have shed no light on the motive for the Boston Marathon bombing and said it was unclear whether any terrorist organizations had a hand in it.

The FBI was swamped with tips after the release of the photos — 300,000 every minute by one estimate — but what role those played in the overnight clash was unclear. State Police spokesman Dave Procopio said police realized they were dealing with the bombing suspects based on what the two men told a carjacking victim during their getaway attempt.

Exactly how the long night of crime began was marked by conflicting reports. But police said the brothers carjacked a man in a Mercedes-Benz in Cambridge, just across the Charles River from Boston, then released him unharmed at a gas station.

They also shot to death a Massachusetts Institute of Technology police officer, 26-year-old Sean Collier, while he was responding to a report of a disturbance, investigators said.

The search for the Mercedes led to a chase that ended in Watertown, where authorities said the suspects threw explosive devices from the car and exchanged gunfire with police. A transit police officer, 33-year-old Richard Donohue, was shot and critically wounded, authorities said.

Dzhokhar Tsarnaev somehow slipped away. He ran over his already wounded brother as he fled by car, according to two law enforcement officials who spoke on condition of anonymity because they were not authorized to discuss the investigation.

Tamerlan Tsarnaev died at a Boston hospital after suffering what doctors said were multiple gunshot wounds and a possible blast injury.

Based on reporting by The Associated Press.

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