WATERTOWN, Mass. -- The two suspects in the Boston Marathon bombing killed an MIT police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday as thousands of officers swarmed the streets in a manhunt that all but paralyzed the Boston area.
The suspects were identified by law enforcement officials and a family member as Dzhokhar and Tamerlan Tsarnaev, brothers from a Russian region near Chechnya, which has been plagued by an Islamic insurgency that has carried out deadly bombings. They lived near Boston and had been in the U.S. for about a decade, an uncle said.
Tamerlan Tsarnaev, a 26-year-old who had been known to the FBI as Suspect No. 1 and was seen in surveillance footage in a black baseball cap, was killed overnight, officials said. His 19-year-old brother -- dubbed Suspect No. 2 and seen wearing a white, backward baseball cap in the images from Monday's deadly bombing at the marathon finish line -- escaped.
The law enforcement officials spoke on condition of anonymity because of the sensitivity of the unfolding case.
Authorities in Boston suspended all mass transit and warned close to 1 million people in the entire city and some of its suburbs to stay indoors as the hunt went on. Businesses were asked not to open. People waiting at bus and subway stops were told to go home.
From Watertown to Cambridge, police SWAT teams, sharpshooters and FBI agents with armored vehicles surrounded various buildings as police helicopters buzzed overhead.
"We believe this man to be a terrorist," said Boston Police commissioner Ed Davis, who urged all 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."
The bombings on Monday killed three people and wounded more than 180 others, tearing off limbs in a spray of shrapnel and instantly raising the specter of another terrorist attack on U.S. soil.
The endgame -- at least for Suspect No. 1 -- came just hours after the FBI released photos and video of the two young men at the finish line and appealed to the public for help in identifying and capturing them. Tips came pouring in to the FBI immediately, but exactly how authorities managed to close in on the two was not immediately disclosed.
The men's uncle, Ruslan Tsarni of Montgomery Village, Md., told The Associated Press that the men traveled here together from the Russian region near Chechnya.
Their father, Anzor Tsarnaev, said in a telephone interview with The Associated Press from the Russian city of Makhachkala that his younger son, Dzhokhar, is "a true angel."
"Dzhokhar is a second-year medical student in the U.S. He is such an intelligent boy. We expected him to come on holidays here," the father said.
According to multiple media reports, Tamerlan Tsarnaev had at one time attended Bunker Hill Community College but had taken time off to train as a boxer for a Golden Gloves competition in Salt Lake City, Utah. He had won a number of amateur regional title bouts, according to multiple media outlets, and had hopes of boxing for the U.S. Olympic team.
Larry Aaronson, a former teacher at Cambridge Rindge and Latin, told The Boston Globe that he knew Dzhokhar.
"If someone were to ask me what the kid was like, I would say he had a heart of gold," he said. "He was as gracious as possible."
While his younger brother was described as someone who fit in and was friendly, Tamerlan Dzhokhar wrote in a photo essay, "I don't have a single American friend. I don't understand them."
The White House said President Barack Obama was being briefed on developments overnight by Lisa Monaco, his assistant for homeland security and counterterrorism.
The images released by the FBI depict the two young men walking one behind the other near the finish line of the marathon. Richard DesLauriers, FBI agent in charge in Boston, said Suspect No. 2 in the white hat was seen setting down a bag at the site of the second of two deadly explosions.
Authorities said surveillance tape recorded late Thursday showed Suspect No. 2 during a robbery of a convenience store in Cambridge, near the campus of MIT, where a university police officer identified as 26-year-old Sean Collier was shot to death while responding to a report of a disturbance, said State Police Col Timothy Alben. Collier's brother, Andrew, is a machinist in the Hendrick Motorsports engine department.
From there, authorities said, the two men 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 about 1 a.m. 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 a.m. 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.
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."
He said he could smell something burning and advanced to check it out, but area residents at their windows yelled at him, "Hey, it's gunfire! Don't go that way!"
Doctors at a Boston hospital where Suspect No. 1 died said they treated a man with a possible blast injury and multiple gunshot wounds.
In the past, insurgents from Chechnya and neighboring restive provinces in the Caucasus have been involved in terror attacks in Moscow and other places in Russia.
Information from The Associated Press and ESPNBoston.com was used in this report.