Man seen in white baseball cap still at large

Two suspects in the Boston Marathon bombing - identified as coming from a Russian region near Chechnya - killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during their getaway attempt in a long night of violence that left one of them dead and another still at large on Friday, authorities said.

Two law enforcement officials told AP the two suspects, who has been identified by national security officials as brothers Dzhokhar A. Tsarnaev, 19, and Tamerlan Tsarnaev, 26, had been living legally in the United States for at least one year.

A law enforcement intelligence bulletin obtained by AP identified the surviving bomb suspect as Dzhokhar.

In Boston, still on edge over the attack on the marathon, and its western suburbs, authorities suspended mass transit and urged people to stay indoors as they searched for Dzhokhar, the man seen wearing a white baseball cap on surveillance footage from Monday's deadly bombing at the marathon finish line.

"We believe this man to be a terrorist," said Boston Police Commissioner Ed Davis, who urged nearby residents to stay in their homes unless police are present. "We believe this to be a man who's come here to kill people."

Authorities urged residents to stay indoors in Watertown, Cambridge and other western suburbs, as well as some western neighborhoods of Boston. Mass transit was shut down, and businesses were asked not to open on Friday. At least a quarter of a million people live in those suburbs. People waiting at bus and subway stops were told to go home.

All modes of public transport were shut down, including buses, subways, commuter rail and boats, said Joe Pesaturo, spokesman for the Massachusetts Bay Transportation Authority.

The clashes with police began only a few hours after the FBI released photos and videos of the two young men, who were seen carrying backpacks as they mingled among marathon revelers. The bombings on Monday killed three people and wounded more than 180 others, and authorities revealed the images to enlist the public's help finding the suspects.

The images released by the FBI depict two young men, each wearing a baseball cap, walking one behind the other near the finish line. Richard DesLauriers, FBI agent in charge in Boston, said the suspect in the white hat was seen setting down a bag at the site of the second of two deadly explosions.

Authorities said surveillance recorded on Thursday showed Dzhokhar during a robbery of a convenience store in Cambridge, near the campus of MIT, where a university police officer was killed while responding to a report of a disturbance, said State Police Col Timothy Alben. The officer died of multiple gunshot wounds.

From there, authorities say, the two bombing suspects carjacked a man in a Mercedes-Benz, keeping him with them in the car for half an hour before releasing him at a gas station in Cambridge. The man was not injured.

The search for the vehicle 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 was seriously injured during the chase, authorities said.

In Watertown, witnesses reported hearing multiple gunshots and explosions at about 1 am on Friday. Dozens of police officers and FBI agents were in the neighborhood and a helicopter circled overhead.

Watertown resident Christine Yajko said she was awakened at about 1:30 am by a loud noise, began to walk to her kitchen and heard gunfire.

"I heard the explosion, so I stepped back from that area, then I went back out and heard a second one," she said. "It was very loud. It shook the house a little."

She said a police officer later knocked on her door and told her there was an undetonated improvised explosive device in the street and warned her to stay away from the windows.

"It was on the street, right near our kitchen window," she said.

Yajko said she never saw the suspect who was on the loose and didn't realize the violence was related to the marathon bombings until she turned on the TV and began watching what was happening outside her side door.

State police spokesman David Procopio said, "The incident in Watertown did involve what we believe to be explosive devices possibly, potentially, being used against the police officers."

Boston cab driver Imran Saif said he was standing on a street corner at a police barricade across from a diner when he heard an explosion.

"I heard a loud boom and then a rapid succession of pop, pop, pop," he said. "It sounded like automatic weapons. And then I heard the second explosion."

MIT said that after the 10:30 pm shooting police were sweeping the campus in Cambridge and urged people to remain indoors. They urged people to stay away from the Stata Center, a mixed-use building with faculty offices, classrooms and a common area.