An armed 26-year-old man under constant surveillance by an anti-terrorism task force was shot and killed by an FBI agent and a Boston police officer in Roslindale this morning after he came at them with a military-style knife, authorities say.

The suspect was identified by police as Usaama Rahim of Roslindale.

"He was on foot, under surveillance," Boston Police Commissioner William Evans said. "The officers have been surveilling him and again they wanted to speak to him … and he turned and our officers gave several commands for him to drop the weapon and unfortunately he came at the officers and they did what they were trained to do and that's never an easy decision for any officer to make."

One FBI agent and one BPD officer fired, FBI Special Agent in Charge Vincent B. Lisi said.

Evans said "the level of alarm" had them want to question Rahim today.

Lisi added task-force members — who had Rahim under 24-hour watch — wanted to "interview him and talk to him about his intentions and some other matters." At the time, Lisi added, Rahim was considered armed and dangerous.

Evans said a video shows Rahim "coming at the officers" as police retreated telling him to "drop the knife!" They then shot him twice, once in the torso and abdomen.

Rahim was declared dead at Brigham & Women's Hospital, Evans said.

Boston police Lt. Mike McCarthy said the suspect is of Middle Eastern descent and had been under surveillance "for weeks," including this morning.

In another joint police-FBI case, a triple-decker in Everett on Linden Street was also raided today and a man was arrested in connection with the investigation, a law enforcement official said. The suspect's name has not been released because he is still being booked. Lisi declined to comment on any possible link to Everett and Roslindale.

Authorities were seen going in and out of the Everett house before the man was taken out. Men in military garb were also seen walking up to the house.

The FBI is also at a house in Warwick on Aspinet Drive, which is also tied to the case, the law enforcement source told the Herald.

Police and FBI agents were also seen today at the Stony Brook apartment complex in Roslindale, near the shooting scene, searching a unit at the Blue Ledge Drive complex.

An eyewitness who declined to give his name said the suspect was armed with a knife and had a "scabbard" when he lunged at police and was shot.

Dozens of police officers have swarmed the small shopping plaza and officers are seeking surveillance video from a nearby Dunkin' Donuts, employees said.

A mother and her 10-year-old son heard gunfire this morning and saw the aftermath of the police shooting in Roslindale from a second-story window across the street from the crime scene, the mother said.

Luz Garces, 47, said she was in her kitchen this morning when she heard a "couple" of gunshots, describing two or three rounds, followed by her young son entering the kitchen telling her, "'Mommy, Mommy, there was a shooting at the CVS."

Garces said at the time, she wasn't sure if she had heard gunfire and told her son it could have been a car that made the noises. But the child insisted and took her to his bedroom window overlooking the CVS as marked and unmarked cars were racing to the scene and she saw a man lying in the middle of the parking lot on the ground, moving his legs, she said.

The boy told Garces he saw two men near the man who was shot and heard orders of "stand back or don't move," Garces said her son told her. She said she could not see a knife from the distance and her son didn't mention a knife. She said she thought her son didn't witness the shooting, but its aftermath after the shots rang out.

Garces said she worries about her son seeing such a traumatic incident.

"He said, 'Mommy, why do people get in a shooting?' I told him some people get in trouble by themselves. I will talk to him when he gets back from school."

Garces also said there had to be many witnesses including several by a bus stop near the CVS. She said the shooting frightened some, noting she saw a woman crying by an adjacent Burger King.

The FBI agent and officer involved are being monitored for stress, Evans said.

"I'm leaving all comments to the FBI and the Boston Police Department," said Boston Mayor Martin J. Walsh, who is at the scene. "It's an ongoing investigation so I'm not going to comment."

Laurel J. Sweet contributed.

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