Boston attacks: breaking news updated: CBS NEWS

1Boston bombing suspect dead; massive manhunt for 2nd, the "white hat" suspect

"Suspect 1" and "Suspect 2" in the Boston Marathon bombings, as identified by the FBI, are seen standing together before the blasts in an image released by the FBI. / FBI/CBS

Updated 6 a.m. ET

WATERTOWN, Mass. One of two suspects in the Boston Marathon bombing is dead and a massive manhunt is under way for another, authorities said early Friday.

Boston Police Commissioner Edward Davis tweeted that the second at large is the one seen in the white hat in images released by the FBI Thursday of the Boston Marathon suspects. Davis says he is "armed and dangerous."

Massachusetts State Police Superintendent Timothy Alben told reporters the situation is "grave."

Residents of Watertown, a Boston suburb, have been advised to keep their doors locked and not let anyone in. The Boston Police Department warned residents to "stay home," and barred any vehicles from entering or leaving Watertown. Public transit in the area has been suspended.

The Middlesex district attorney said the two men are suspected of killing a Massachusetts Institute of Technology police officer on campus late Thursday, 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.

/ Boston Police

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 while the other escaped.

CBS News has learned the dead suspect had multiple gunshot wounds.

"We believe this to be a terrorist," Boston police Commissioner Ed Davis said. "We believe this to be a man who came here to kill people."

The FBI said it is working with local authorities to determine what happened.

The MIT shooting on the Cambridge campus Thursday night was followed gunfire and explosions in Watertown, about 10 miles west of Boston.

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, multiple gunshots and explosions were heard 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.

/ FBI/CBS

"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.

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