Sources have told the Associated Press that that the Boston bomb suspects are from a Russian region near Chechnya and have lived in the US for at least one year.
The news organisation identified the surviving suspect as Dzhokhar A Tsarnaev, 19, of Cambridge, Massachusetts.
US police are continuing to carry out a massive manhunt for a suspect after the second suspect was killed in a shootout with officers.
Police are locking down some neighbourhoods in Boston and its western suburbs as they search for the remaining suspect from marathon surveillance footage.
Authorities urged residents in Watertown, Cambridge and other towns west of Boston, as well as the Allston-Brighton neighbourhoods of western Boston, to stay indoors.
All public transport was shut down and businesses were asked not to open. People waiting at bus and subway stops were told to go home.
“We believe this to be a terrorist,” said Boston Police Commissioner Ed Davis of the suspect on the loose.
“We believe this to be a man who’s come here to kill people. We need to get him in custody.”
The Middlesex district attorney said the two men are suspected of killing a Massachusetts Institute of Technology police officer on campus in Cambridge in a late-night shooting, then stealing a car at gunpoint and later releasing its driver unharmed.
Hours earlier, police had released photos of the marathon bombing suspects and asked for the public’s help finding them.
A new photo of the suspect on the loose was released later showing him in a grey hoodie sweatshirt. It was taken at a 7-Eleven store in Cambridge, just across the Charles River from Boston.
The first images were released hours after President Barack Obama and first lady Michelle Obama attended an interfaith service at a Roman Catholic cathedral in Boston to remember the bombing victims.
Authorities say the suspects threw explosives from the car as police followed it into Watertown.
The suspects and police exchanged gunfire, and one of the wanted men was critically injured and later died at a hospital while the other escaped.
The FBI said it was working with local authorities to determine what happened.
Doctors at Beth Israel Deaconess Medical Centre in Boston where the suspect was taken and later died said they treated a man with a possible blast injury and multiple gunshot wounds.
They wouldn’t say if the patient they treated, who came in with police, was the suspect in the black hat.
The MIT shooting on the Cambridge campus night was followed by reports of gunfire and explosions in Watertown, about 10 miles west of Boston.
The MIT officer had been responding to report of a disturbance when he was shot multiple times, according to a statement from authorities. There were no other victims.
In Watertown, witnesses reported hearing multiple gunshots and explosions at about 1 am local time. Dozens of police officers and FBI agents were in the neighbourhood and a helicopter circled overhead.
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!”
MIT said right after the 10:30 pm shooting that police were sweeping the campus in Cambridge and urged people to remain indoors.
They urged people urged to stay away from the Stata Centre, a mixed-use building with faculty offices, classrooms and a common area.
Hours later, MIT, the prestigious university with about 11,000 students, said the campus was clear but the shooter was still on the loose.