WATERTOWN, Mass. (AP) - Authorities say one of two suspects in the Boston Marathon bombing is dead and a massive manhunt is underway for another.
They have released a new photo showing the man believed to be a suspect in the bombings, shown in earlier footage in a white hat.
AP sources identify the man as Dzhokhar Tsarnaev, 19, of Cambridge, Mass. He is the bombing suspect who is still alive. The other bombing suspect was killed overnight in an exchange of gunfire with police.
In the new photo, he's wearing a gray hoodie sweatshirt pulled up on his head. The photo was taken at a 7-Eleven in Cambridge, just across the river from Boston.
The sources also say the men are brothers from a region of Russia near Chechnya, and have lived in the U.S. for at least a year.
The entire city of Boston is on lockdown and residents are asked to "shelter in place," as police search for the remaining suspect in the marathon bombings.
Authorities urged residents in Watertown, Newton, Waltham, Belmont, Cambridge and the Allston-Brighton neighborhoods of Boston to stay indoors. All mass transit is shut down.
A no fly zone has been enacted over Watertown.
The Middlesex district attorney says the two men are suspected of killing an MIT police officer at the college late Thursday, then stealing a car at gunpoint and later releasing its driver unharmed. Hours earlier, police had released photos of the bombing suspects and asked for the public's help finding them.
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.
Police say the suspect on loose is a "terrorist" who "came here to kill people."
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
Police say one of two suspects in the shooting of a Massachusetts Institute of Technology police officer is dead and a massive manhunt is underway for another, who is believed to be tied to the Boston Marathon bombing.
Shortly after the MIT officer was shot dead Thursday night, police got a report of a carjacking in Cambridge, just outside Boston. One of the two suspects in that officer's shooting was killed. Police say of the at-large suspect, "We believe this to be a terrorist."
The FBI said it is working with local authorities to determine what happened.
The MIT shooting on the Cambridge campus Thursday night was followed by reports of violence in nearby Watertown, about 10 miles west of Boston.
State police spokesman David Procopio had said there was a "strong possibility" the incidents are related.
The MIT officer had been responding to report of a disturbance Thursday night 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 a.m. Friday. Dozens of police officers and FBI agents were in the neighborhood 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 p.m. 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 Building, a mixed-use building with faculty offices, classrooms and a common area.
Hours later, MIT, which has about 11,000 students, said the campus was clear but the shooter was still on the loose.