Boston Bombing Suspects Identified As Brothers From Chechnya. 1 Killed, Police Searching For 2nd Man [Video/Pics]

Video And Pics After The Jump

Authorities shot and killed one suspect in Monday’s Boston Marathon bombings and were conducting a massive hunt for a second suspect in Watertown, Mass., Friday morning following a chaotic night that left one police officer dead and another critically wounded in the Boston suburbs.

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 still at large was identified by law enforcement authorities as Dzhokhar A. Tsarnaev, 19, of Cambridge.

The brothers’ alleged motive in the bombings, which killed three people and injured more than 170, remains unknown.

Dzhokhar A. Tsarnaev (19) and Tamerlan Tsarnaev (26)

Law enforcement officials said they believe the suspect may be strapped with explosives, and they are taking extreme measures during a massive manhunt to avoid further loss of life.

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

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

Residents of Watertown and several surrounding suburbs — Newton, Cambridge, Waltham and elsewhere — were asked to stay inside. 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.

The mayhem began about 10:30 p.m. when a Massachusetts Institute of Technology 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 nearby in Cambridge. The men were later identified by police as the two suspects they were searching for in Monday’s deadly marathon bombings, which killed three people and injured more than 170.

Police began a search for a Mercedes SUV that had been taken at gunpoint by the two males and was later spotted in neighboring Watertown, the district attorney’s office said.

Watertown police officers located the vehicle and after stopping it, exchanged gunfire with two men in a residential neighborhood. 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.

A transit police officer was critically wounded in the firefight, authorities said.

Police fatally injured one suspect during the firefight, the district attorney’s office said. He was taken to Beth Israel-Deaconness Hospital, where he was pronounced dead, doctors there said.

The man, whom authorities later identified as the suspect pictured in a black baseball cap in photos released Thursday evening, 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.

A second man fled the vehicle on foot, which prompted the massive search by authorities. They created a 20-block perimeter in a section of Watertown and advised residents to lock their doors and only answer to uniformed police officers.

At 6:30 a.m., a time when residents of the town would normally be waking up and starting to head to work or school, the streets were deserted save for a massive police presence.

“We have an active search going on by tactical teams. He’s considered armed and dangerous,” Col. Timothy P. Alven, said at a televised early morning news conference.

Investigators believe the man who is at large is suspect No. 2 in the photos, which were culled from surveillance footage shortly before Monday’s attack. He was pictured wearing a white hat on Monday.

At the news conference, officials released a surveillance photo from a convenience store in Cambridge taken before the fatal shooting at MIT. That photo showed a man police also believe is suspect No. 2. He was wearing a gray hooded sweatshirt late Thursday night.

Law enforcement officials said they are shutting down the transportation system and are instructing residents to stay indoors because they believe suspect No. 2 may be strapped with explosives.

At the site of the shootout with police, bomb squad technicians worked to render secure several devices safe that had been thrown after the stolen SUV was stopped, Alven said.

Sari Horwitz in Washignton and Annie Gowen in Watertown, Mass. contributed to this report.

Dead Suspect Died of Blast and Gunshot Wounds

Wall Street Journal report

Al Jazeera English report

Boston city in lockdown: second shooting suspect still at large

Police: Suspect Is 'Here to Kill People'

Associated Press report