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From our Globe Team:

www.boston.com

By Wesley Lowery, Akilah Johnson, Eric Moskowitz and Lisa Wangsness, Globe Staff

WATERTOWN — One suspect in Monday’s Boston Marathon bombings has been apprehended, according to an official with knowledge of the investigation. Another appears to remain on the loose in Watertown after a firefight with police. A 20-block perimeter was established as authorities searched for him.

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 frenetic chase into Watertown. Hundreds of police officer flooded Cambridge and Watertown.

A spokesman for the MBTA said that a transit officer had been shot and was in serious condition.

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

Massachusetts State Police spokesman David Procopio said early Friday that the violent events at MIT and Watertown appeared to be connected, and that federal authorities were investigating whether the violence of Thursday night and Friday was connected to the marathon bombings.

At least one of the suspects in Watertown appeared to be a man in his 20s.

FBI agents were on the scene.

“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.

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 Route 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 gunshots and didn’t know what to do.

“She was saying they’re running down the street shooting,” Antonucci said, standing outside 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 doors and windows open, and feed stray cats.

Standing at Quimby Street and Nichols Avenue 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.

“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 to Cambridge when he heard a series of loud noises that he said “sounded like fireworks.” He said he biked toward the sounds, thinking they were 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.”

Dan MacDonald, who lives on Bigelow Avenue and Mount Auburn Street, near Watertown Square, said he was watching TV and talking with his girlfriend when they began hearing sirens -- just a few at first, then more -- “maybe five or seven, racing at this point.” Then in the distance they heard gunshots, about 15, he said, within 10 seconds.

“I kind of ran downstairs and came outside,” he said. “They were coming from the Arsenal Street area up Bigelow Avenue. There were about 10 cop cars, they took a left on Mount Auburn Street heading toward Galen Street.”

The bedlam in Watertown was preceded by a spasm of violence in Kendall Sqaure, in Cambridge.

An MIT officer, who has not been identified, was shot multiple times at 10:48 p.m. Main and Vassar streets, near Building 32, better known as the renowned the Stata Center. No one else was hurt, and no ­arrests had been made by early Friday.

That shooting sparked a massive manhunt, which fanned out from ­Kendall Square over an area that had endured a tragic and tumultuous week, in the aftermath of the fatal explosions at the marathon.

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, which was later lifted.

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.

The grievously wounded officer was taken to Massachusetts General Hospital, where he was pronounced dead. At the hospital, family members of the officer shot and killed declined to comment or give the officer’s name. About a dozen gathered outside the hospital’s emergency room, hugging and consoling one another through the night.

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.

Early Friday, MIT issued a statement about the death of the officer. “MIT is heartbroken by the news that an MIT Police officer was shot and killed in the line of duty on Thursday night on campus. Our thoughts are now with the family.”

Marcella Bombardieri, ­Brian MacQuarrie, Martine Powers, Maria Sacchetti, and Milton J. Valencia of the Globe staff and correspondents Jeremy C. Fox, Haven Orecchio-Egresitz, Jaclyn Reiss, and Gal Tziperman Lotan contributed to this report.