One of two suspects in the deadly Boston Marathon bombing is dead and a massive manhunt is under way for another, officials said today.
Residents of Watertown, a suburb just outside Boston, have been advised by police to keep their doors locked and not let anyone in.
Public transport has been suspended in the Boston area as the manhunt continues, authorities said.
People in some areas were also urged to stay indoors and businesses were urged not to open.
"We believe this to be a terrorist," said Boston Police Commissioner Ed David.
"We believe this to be a man who's come here to kill people. We need to get him in custody."
A law enforcement intelligence bulletin obtained by the AP identified the surviving suspect as Dzhokhar A. Tsarnaev, 19, of Cambridge, Massachusetts.
Two law enforcement officials said Tsarnaev and the other suspect who was not immediately identified have been living legally in the US for at least one year. Russia's North Caucasus region has been plagued by an Islamic insurgency stemming from separatist wars in Chechnya.
Doctors at a Boston hospital where the suspect was taken and later died are saying they treated a man with a possible blast injury and multiple gunshot wounds.
Doctors and officials at Beth Israel Deaconess Medical Centre wouldn't say if the patient they treated, who came in with police, was the suspect in the black hat from marathon surveillance footage.
An emergency room doctor says he heard gunshots near his home in Watertown and knew he needed to get back to the hospital.
Doctors and hospital officials say they don't have any information on the identity of the dead patient.
The Middlesex district attorney said the two men are suspected of killing a Massachusetts Institute of Technology police officer on campus late last night, then stealing a car at gunpoint and later releasing its driver unharmed.
Police have released a CCTV image of a man - referred to as 'Suspect number two' - taken from a convenience store last night. Police say the man is a suspect in the Boston Marathon bombing and was wearing a white hat in photos taken at the race
Hours earlier, police had released photos of the marathon bombing suspects and asked for the public's help finding them.
A new photo of Tsarnaev was released later showing him in a grey-hooded sweatshirt. It was taken at a 7-Eleven store in Cambridge, just across the river from Boston.
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 suspects 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.
The MIT shooting on the Cambridge campus was followed by reports of gunfire and explosions in Watertown, about 10 miles (16 kms) 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 the Middlesex district attorney's office and Cambridge police. It said there were no other victims.
In Watertown, witnesses reported hearing multiple gunshots and explosions at about 1 am local time today. 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 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.