The two suspects were identified by law enforcement officials as brothers from Chechnya. The surviving suspect was identified as Dzhokhar A Tsarnaev, 19, of Cambridge, Massachussets, a law enforcement official said. The one who was killed was identified as his brother, Tamerlan Tsarnaev, 26. Authorities were investigating whether the dead man had a homemade bomb strapped to his body when he was killed, two law enforcement officials said.
The manhunt for the surviving bombing suspect sent the Boston region into the grip of a security emergency: Residents of the city and the surrounding area were urged to stay indoors, as hundreds of police officers conducted a manhunt and all public transit services were suspended.
Colonel Tim Alben of the Massachusetts State Police said investigators believed that the two men were responsible for the death of a Massachusetts Institute of Technology police officer and the shooting of an officer with the Massachusetts Bay Transportation Authority, the region's transit authority.
"We believe these are the same individuals that were responsible for the bombing on Monday at the Boston Marathon," he said.
Officials said the two men were from Chechnya, a long-disputed, predominantly Muslim territory in southern Russia that fought two bloody wars in the 1990s against Russian authorities.
The family lived briefly in Makhachkala, the capital of the Dagestan region, near Chechnya, before moving to the United States, said a school administrator there. Irina V Bandurina, secretary to the director of School No. 1, said the Tsarnaev family left Dagestan for the United States in 2002 after living there for about a year. She said the family - parents, two boys and two girls - had lived in the Central Asian nation of Kyrgyzstan previously.
Both brothers have substantial presence on social media. On Vkontakte, Russia's most popular social media platform, the younger brother, Dzhokhar, describes his worldview as "Islam" and, asked to identify "the main thing in life," answers "career and money." He lists a series of affinity groups relating to Chechnya, and lists a verse from the Quran, "Do good, because Allah loves those who do good."
The older brother left a record on YouTube of his favorite clips, which included Russian rap videos, as well as testimonial from a young ethnic Russian man titled "How I accepted Islam and became a Shiite," a clip titled "Seven Steps to Successful Prayer."
Alvi Karimov, the spokesman for Ramzan Kadyrov, leader of Chechnya, said the Tsarnaev brothers have not lived in Chechnya for many years. He told the Interfax news service that, according to preliminary information, the family: "moved to a different region of the Russian Federation from Chechnya many years ago. Then the family lived for a long time in Kazakhstan, and from there moved to the United States, where the members of the family received residency permits."
"In such a way, the figures who are being spoken about did not live in Chechnya at a mature age, and if they became 'bad guys,' then this is a question that should be put to the people who raised them," he said.
Early Friday, a virtual army of heavily armed law enforcement officers was still going through houses in Watertown, outside of Boston, one by one in a search for the second suspect. Police had blocked off a 20-block residential area and urged residents emphatically to stay inside their homes and not answer their doors.
Boston Police Commissioner Edward Davis said: "We believe this to be a man who's come here to kill people, and we need to get him in custody."
With gunfire ricocheting around the tranquil neighbourhood, residents were later told to go into their basements and stay away from windows.
The pursuit began after 10 pm Thursday when two men robbed a 7-Eleven near Central Square in Cambridge. A security camera caught a man identified as one of the suspects, wearing a gray hoodie.
About 10:30, police received reports that a campus security officer at MIT was shot while he sat in his police cruiser. He was found with multiple gunshot wounds, according to a statement issued by Middlesex Acting District Attorney Michael Pelgro, Cambridge Police Commissioner Robert Haas and MIT Police Chief John DiFava. The officer was taken to Massachusetts General Hospital, where he was pronounced dead.
A short time later, police received reports of an armed carjacking of a Mercedes SUV by two men in Cambridge, the statement said.
"The victim was carjacked at gunpoint by two males and was kept in the car with the suspects for approximately a half-hour," the statement said.
He was later released, uninjured, at a gas station in Cambridge.
Police immediately began to search for the vehicle and pursued it into Watertown. During the chase, "explosive devices were reportedly thrown from car by the suspects," the statement said, and the suspects and police exchanged gunfire.
A Watertown resident, Andrew Kitzenberg, 29, said he looked out his third-floor window to see two young men of slight build in jackets engaged in "constant gunfire" with police officers. A police SUV "drove towards the shooters," he said, and was shot at until it was severely damaged. It rolled out of control, Kitzenberg said, and crashed into two cars in his driveway.
The two shooters, he said, had a large, unwieldy bomb that he said looked "like a pressure cooker."
"They lit it, still in the middle of the gunfire, and threw it. But it went 20 yards at most."
It exploded, he said, and Tamerlan Tsarnaev ran toward the gathered police officers. He was tackled, but it was not clear if he was shot, Kitzenberg said.
The explosions, said another resident, Loretta Kehayias, 65, "lit up the whole house. I screamed. I've never seen anything like this, never, never, never."
Meanwhile, Dzhokhar A. Tsarnaev, said Kitzenberg, got back into the SUV, turned it toward officers and "put the pedal to the metal."
The car "went right through the cops, broke right through and continued west."
The two men left "a few backpacks right by the car, and there is a bomb robot out there now."
Police had told residents to stay away from their windows, he said.
During this exchange, an MBTA police officer was seriously injured and taken to the hospital.
At the same time, Tamerlan Tsarnaev was critically injured with multiple gunshot wounds and taken to Beth Israel Deaconess Hospital in Boston, where he was pronounced dead at 1:35 am.
A doctor who works at Beth Israel, and who lived in the area of the chase and shootout, said he was working at home around 1 am when he heard the wailing sirens. He said at a news conference at Beth Israel that he recognized that something was wrong and alerted his emergency room to prepare for something.