The suspects were involved in the MIT shooting and Watertown violence overnight, police say.
Authorities say one of two suspects in the Boston Marathon bombing is dead and a massive manhunt is underway for another.
Police are locking down some neighborhoods in Boston and its western suburbs as they search for the remaining suspect. All mass transit also was shut down.
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 the loose is a "terrorist" who "came here to kill people."
Shortly after the MIT officer was shot dead, police got a report of a carjacking in Cambridge, just outside Boston.
The FBI said it is working with local authorities to determine what happened.
The MIT shooting on the Cambridge campus was followed by reports of violence in nearby 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 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. today. Dozens of police officers and FBI agents were in the neighborhood and a helicopter circled overhead.
State police spokesman David Procopio said early this morning, "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.