WATERTOWN, MASSACHUSETTS – A massive manhunt was under way Friday morning in Boston and its surburbs, after one suspect in the deadly Boston Marathon bombings died in a confrontation with police and the second was identified as a 19-year-old immigrant from Kyrgystan who, a classmate said, attended high school in Cambridge, Massachusetts.

Boston, Watertown and several other suburbs were in a state of lockdown, with mass transit canceled, schools and business closed and residents ordered to stay indoors, authorities said.

Law enforcement officials said they believe the at-large suspect, Dzhokhar Tsarnaev, may be strapped with explosives. They are taking extreme precautions in an effort to avoid further loss of life.

A campus security officer at the Massachusetts Institute of Technology was killed in a confrontation with the suspects Thursday night, and a transit officer was critically wounded, police said.

“This situation is grave. We are here to protect public safety,” Police Commissioner Ed Davis said. “We believe this to be a terrorist. We believe this to be a man here to kill people.”

The suspects — introduced to the world via photos and video footage Thursday night — were identified as brothers, law enforcement officials said Friday morning. The one who was killed was identified as Tamerlan Tsarnaev.

The brothers’ alleged motive in the bombings, which killed three people and injured more than 170, remains unknown, but their family appears to have immigrated from the Southern Russian republic of Chechnya, and two law enforcement officials said there is a “Chechen connection” to the bombings.

Dzhokhar Tsarnaev was born in Kyrgystan, law enforcement authorities said. He has a Massachusetts driver’s license. Tamerlan Tsarnaev was born in Russia and became a legal U.S. resident in 2007.

All public transportation was shut down in the greater Boston area Friday morning, officials said, and no vehicle traffic was permitted in or out of Watertown during the massive manhunt.

Residents of Boston, Watertown, Newton, Waltham and elsewhere were asked to stay inside, with their doors locked. Colleges and universities announced they would close for the day, and businesses were instructed not to open.

Thousands of officers searched house-to-house, and parts of Watertown were being evacuated.

A Massachusetts State Police spokesman says police closed down a stretch of Norfolk Street in Cambridge, where they think Dzhokhar Tsarnaev may be located. “We don’t know if he’s there. There is a possibility the suspect is there,” the spokesman said.

In Washington, President Obama was briefed by his counterterrorism adviser, Lisa Monaco.

The mayhem began at about 10:30 p.m. when an MIT campus police officer responded to a disturbance. That officer was found fatally wounded inside his vehicle on the campus in Cambridge, according to the Middlesex District Attorney’s office.

The shooting launched a massive police response. A short time later, authorities received reports of an armed carjacking by two males of a Mercedes SUV, on Third Street in Cambridge. The driver of the car was forced to stay in the vehicle for about 30 minutes, police said, then released at a gas station on Memorial Drive. He was unharmed.

Police began searching for the Mercedes SUV, which was spotted in a residential neighborhood of nearby Watertown, the district attorney’s office said.

Watertown police officers located the vehicle and after stopping it, exchanged gunfire with the two men. During the firefight, authorities said, multiple explosive devices were thrown from the vehicle and some exploded, which led to panic and concern in the town.

Richard H. Donahue Jr., a transit police officer, was critically wounded in the firefight, authorities said. Donahue, 33, has been a member of the force for three years. He is being treated at a local hospital.

The first suspect — Tamerlan Tsarnaev, who was pictured in a black baseball cap in photos released Thursday evening was fatally injured, law enforcement officials said. He was taken to Beth Israel-Deaconess Medical Center, where he was pronounced dead, doctors there said.

The man had been shot multiple times in the torso and also sustained injuries from some sort of explosives, doctors at the hospital said. He was in cardiac arrest when he arrived at the hospital, and could not be revived.

Dzhokhar Tsarnaev, or suspect No. 2, according to authorities — fled the vehicle on foot, which prompted the massive search.

“We have an active search going on by tactical teams. He’s considered armed and dangerous,” said Col. Timothy P. Alven.

A high school classmate of Dzhokhar Tsarnaev, Deana Beaulieu, described him as a quiet boy who had been on the wrestling team at Cambridge Rindge & Latin High School.

They had attended school together since the 7th grade, first at Cambridge Community Charter School, then the high school, she said. He graduated in 2011.

Tsarnaev lived on Norfolk Street with his family, including an older brother and sister.

State Department officials said the Tsarnaev family appears to have arrived legally in the United States, though they did not specify when they arrived or the type of visas the family members had received. Chechens have dispersed across the former Soviet republics and other countries in the region, but officials said there are not large numbers of them in the United States.

Chechnya has been racked by years of war between local separatists and Russian forces and extensive organized crime since the Soviet Union dissolved in 1991. The extent of the possible connection remained unclear.

Friday morning, the streets in and around Watertown were deserted save for the enormous police presence. Outside the Arsenal Mall, hordes of reporters waited outside a staging area that was taking on the appearance of an armed camp, with State Police marching in formation, dozens of motorcycle police officers and the arrival of two large transit buses filled with police wearing neon safety vests. Agents with the Bureau of Alcohol, Tobacco and Firearms swept the area with a bomb-sniffing dog.

We’ve got every asset that we can possibly muster on the ground right now,” Gov. Deval L. Patrick said. “We are going to need the public to help us help them stay safe.”

Michael Demirdjian, 47, a postal worker from Watertown, said he was on his way back from Logan Airport early Friday when he suddenly found himself surrounded by police cars.

“It was amazing,” he said. “There were police cruisers all around. Thirty to forty cruisers followed us to my house.”

He made it to his house, on Spruce Street, but “it was in the zone and they wouldn’t let us in.”

He said he saw police going from house to house with dogs, searching, the area blazing with flashing emergency lights. Heavily armed police told him he could not enter.

“They said ‘no way.’ I want to go home but it looks like it’s not going to happen,” said Demirdjian, who had been awake all night. “I’d like for them to get this thing under control as soon as possible.”