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Police ordered businesses in the suburb of Watertown and nearby communities to stay closed and told residents to stay inside and answer the door for no one but authorities. Boston authorities advised the same. The city's subway, bus and Amtrak train systems have been shut down. Taxi service across the city was suspended. Every Boston area school is closed.

"It's jarring," said CNN Belief blog writer Danielle Tumminio, who lives in Watertown.

Boston's public transit authority sent city buses to Watertown to evacuate residents while bomb experts combed the surroundings for possible explosives.

Police shot one of the men dead after a wild car chase through Watertown in which authorities say they hurled explosives at pursuing officers.

Police believe the men are the same ones pictured in images released Thursday by the FBI as suspects in the marathon bombing that killed three people Monday.

The men are shown in the images walking together near the marathon finish line.

Several sources told CNN that the dead suspect has been identified as Tamerlan Tsarnaev, 26. The one still being sought is Dzhokhar Tsarnaev, age 19.

The first suspect, the one believed killed by police, appears in the images wearing a dark hat, sunglasses and a backpack. The second suspect, wearing a white cap, is the one who remains at large, police said.

Police warned Watertown residents to lock their homes and stay away from their windows and doors.

Federal, state and local law officers are swarming through Watertown, going door-to-door to track him down, said Massachusetts State Police spokesman Col. Timothy Alben.

Police officers in full body armor, carrying automatic weapons, flooded the area as authorities praised residents for their cooperation.

"We need more time," Alben said. "We're making significant progress up there. But it may take hours to do this."

"This situation is grave." Alben said earlier. "This is a very serious situation that we are dealing with."

The violence began late Thursday with the robbery of a 7/11 convenience store, he said. Soon after, in Cambridge, across the Charles River from Boston, a Massachusetts Institute of Technology police officer was fatally shot while he sat in his car, the Suffolk County District Attorney's Office said in statement. Police believe the bombing suspects were responsible for the shooting.

The same two suspects, according to authorities, then hijacked a car at gunpoint in Cambridge. They released the driver a half-hour later at a gas station.

As police picked up the chase, the car's occupants threw explosives out the windows and shot at officers, according to the district attorney's office.

Officers fired back, wounding one of the men, possibly the person identified by the FBI as suspect No. 1, who is seen in the images released Thursday in a dark cap, sunglasses and wearing a black backpack.

The man died at Beth Israel Hospital. He had bullet wounds and injuries from an explosion, according to officials. The second man apparently escaped on foot.

Richard H. Donohue Jr., 33, a three-year veteran of the transit system police force, was shot and wounded in the incident and taken to a hospital, a transit police spokesman said Friday. The officer's condition was not immediately known.

CNN photographer Gabe Ramirez arrived in Watertown as the chase ended.

"Police were in a standoff with the vehicle just down the hill," Ramirez said. They ordered one suspect out and commanded him to strip down completely naked before putting him in a patrol car, which did not leave the scene.

The man was later released and is not a suspect in the case.

But while the man was being held, FBI agents approached the squad car, and police ordered the man back out of the car. FBI agents questioned him -- still fully undressed -- on the sidewalk.

In an early phase of the lockdown, a man could be seen lying face down on the street with his hands outstretched in front of him and his legs crossed. It is unclear whether this was the man who was arrested and ordered to undress.

Details about suspects

According to a source briefed on the investigation, the suspects involved in the Boston bombing are originally from the Russian Caucasus and had moved to Kazakhstan at a young age before coming to the United States several years ago.

Another source, from federal law enforcement, also told CNN that the man being sought has been in the United States for at least a couple of years.

The man identified as Tamerlan Tsarnaev, the older brother, had studied at Bunker Hill Community College and wanted to become a engineer, the source said. He then took a year off to train as a boxer.

The source told CNN's Deborah Feyerick that a posting on a social media site in his name included the comments: "I don't have a single American friend. I don't understand them."

The source added that it should not be assumed that either brother was radicalized because of their Chechen origins.