WATERTOWN — One suspect in Monday’s Boston Marathon bombings has been captured, according to an official with knowledge of the investigation. Another remains on the loose in Watertown after a firefight with police. Authorities have established a 20-block perimeter as they search for him.

Boston Police Superintendent Daniel Linsky gestured at the scene of a reported shootout in Cambridge

A scene of chaos descended on Cambridge and Watertown late Thursday night and early Friday morning, as police confirmed an MIT police officer was shot and killed, and an apparent carjacking led police on a wild chase into Watertown.

Witnesses in Watertown said they heard explosions. Police officers were screaming about improvised explosive devices.

Authorities would not comment on whether the events were connected to Monday’s Boston Marathon bombings. At least one of the suspects in Watertown appeared to be a man in his 20s.

FBI agents were on the scene in Watertown.

“We are aware of the situation, we are being involved, and we are monitoring,” said an FBI representative who requested anonymity because of not being authorized to speak publicly. The FBI source said early Friday it is “too early to speculate” on a relation to the Marathon bombing.

Dozens of police officers descended on Watertown Square after midnight.

“This is still extremely dangerous,” an FBI agent said. The Cambridge bomb squad arrived in Watertown shortly after 1:30 a.m.

A man in handcuffs was being questioned by the FBI in the back of an ambulance.

Suspect questioned

At Arsenal Court and Arsenal Street in Watertown, an officer bellowed: “Ya gotta get outta here. There’s an active shooter here with an active explosive. Go!”

Peter Jennings, 33, said he was sleeping just before 1 a.m. in his home on Prentiss Street in Watertown when he was awakened by a huge boom.

“It sounded like a stick of dynamite went off,” he said. “I looked out the window, and it was like nothing I’ve ever seen – blue light after blue light after blue light.”

He said more than three dozen emergency vehicles with sirens blaring were heading down Rt. 16 West. He went to the end of his street, where some neighbors were gathering. The air, he said, smelled like “at the end of a fireworks show, like a wick smell.”

“I had a bad feeling because of what happened on Monday,” he said.

John Antonucci’s 79-year-old mother called him hysterical from her home in Laurel Street. She heard about five gun shots and didn’t know what to do.

“She was saying they’re running down the street shooting,” Antonucci said standing outside if yellow police tape. “She was crying so hard I couldn’t understand what she was talking about.”

So he told her: Stay inside the house.

Residents describe the neighborhood as safe and family oriented, where they leave open doors and windows and feed stray cats.

Standing on the corner of Quincy and Nichols as police officers hastily strung up caution tape, Lindsay Gaylord, 25, and Collin Ausfeld, 26, peered over the scene to get a glimpse of their apartment about a block away on Dartmouth Street.

Caution barrier

“I was buying ice cream right there” -- Gaylord pointed to a structure a few steps away, behind the caution tape “just this afternoon.”

Ausfeld stared at the crime scene in front of him, trying to make sense of what he was seeing. As an afterthought, he muttered, “I hope the apartment doesn’t blow up.”

The couple said they moved to the neighborhood in January, leaving behind their Belmont place, because Watertown was closer to the city, and their block was quiet, safe, and friendly.

“After this, I still feel safe on this street,” Gaylord said. “I mean, you just never know with these things.”

Adam Healy, 31, said he stepped outside for a cigarette near one of the shooting scenes in Watertown, when he heard gunfire.

“I just heard tons of gunshots,” he said. “Gunshot, gunshot, gunshot, gunshot. Then I saw an explosion and saw a burst of light in the sky.”

Imran Saif, a cab driver, was parking his car for the night near Dexter and School streets and was preparing to bike home from Cambridge when he heard a series of loud noises that he said “sounded like fireworks.” He said he biked toward the sounds, thinking they fireworks, when people in nearby houses began waving him back, telling him it was gunfire.

“It just sounded like there was automatic weapons going off, and I heard a few explosions,” he said. “They sounded like fireworks, mostly, big fireworks going off -- tons, I’d say. I’m really scared. When I found out it was gunshots, that just knocked the wind out of me.”

Police were demanding that cellphones be turned off.

The MIT officer, who has not been identified, was shot multiple times at 10:48 p.m., according to the Middlesex district attorney’s office. No one else was hurt, and no ­arrests had been made by early Friday.

The manhunt fanned out from ­Kendall Square over an area that has endured a tragic and tumultuous week, in the aftermath of the fatal explosions at Monday’s Boston Marathon. There was no report of a connection between the two events, but the swarm of sirens and circling helicopters rattled a region already on edge.

Police from several agencies were conducting a manhunt for the gunman across the school campus and on the T’s Red Line, according to authorities.

MIT and Cambridge police responded to a report of shots fired at 10:48 p.m. Thursday near Main and Vassar streets, according to the university.

Police officers and canine units swept the campus, and a big swath of Vassar Street was blocked.

The university issued an alert to students and faculty to remain inside.

An eerie quiet descended on the campus as teams of ­police officers combed the campus block by block. SWAT teams were present.

Police checked bushes and alleys and yanked on doors.

Officers from the MIT and Harvard departments, as well as Cambridge and State Police, were present.

Police officer shot to death at MIT campus near Boston

Siddhartha Varshney was walking home from dinner with two friends when they were stopped at the police cordon.

“Initially, we thought they had caught the suspect in the bombing,” the 28-year-old said. But they then learned it was a shooting involving an MIT officer.

“Well, I — honestly — I mean, I can’t think what I make of it. The situation is a little tense,” he said. “And I hope that whoever he is gets caught.”

Few seemed to be out on the campus at the time of the shooting. One professor, standing feet from the police tape, said he came out of his office when he heard a commotion of sirens and saw ­police lights.

At around midnight, a frantic scene was unfolding at Massachusetts General Hospital, where a dozen police cars arrived with sirens on and several women were brought in by police, looking deeply ­upset - Fx