BOSTON » A man who was under 24-hour surveillance by terrorism investigators was shot and killed Tuesday after he lunged with a knife at a police officer and an FBI agent outside a pharmacy, authorities said.

A law enforcement official with knowledge of the investigation into what happened said the man had been making threats against law enforcement. The official wasn’t authorized to release details of the investigation and spoke on the condition of anonymity.

Police Commissioner Williams Evans said members of the Joint Terrorism Task Force approached Usaama Rahim in the city’s Roslindale neighborhood on Tuesday morning to question him about "terrorist-related information" they had received when he went at officers with a large military-style knife.

Evans said officers repeatedly ordered Rahim to drop the knife but he continued to move toward them with it. He said task force members fired their guns, hitting Rahim once in the torso and once in the abdomen. Rahim, 26, was taken to a hospital, where he was pronounced dead.

Evans would not disclose why Rahim was under surveillance. But Evans said a "level of alarm" prompted authorities to try to question him.

"Obviously, there was enough information there where we thought it was appropriate to question him about his doings," Evans said. "He was someone we were watching for quite a time."

Evans later said authorities knew Rahim "had some extremism as far as his views," but he would not confirm media reports that Rahim had been radicalized by online propaganda by the Islamic State group.

Evans said the officers didn’t have their guns drawn when they approached Rahim. He said police have video showing Rahim "coming at officers" while they are backing away.

That account differs from one given by Rahim’s brother Ibrahim Rahim, who said in a Facebook posting that his youngest brother was killed while waiting at a bus stop to go to his job.

"He was confronted by three Boston Police officers and subsequently shot in the back three times," he wrote. "He was on his cell phone with my dear father during the confrontation needing a witness."

Ibrahim Rahim, a former assistant imam at a Boston mosque, said he was traveling to Boston to bury his brother.

The Suffolk district attorney’s office and the FBI said they will investigate Rahim’s shooting, a routine procedure for shootings involving police.

The Council of American-Islamic Relations will monitor the investigation, spokesman Ibrahim Hooper said.

"We have a number of questions," Hooper said. "Why exactly was he being followed? What was the probable cause for this particular stop? Were there any video cameras or body cameras of the incident? How do you reconcile the two versions of the story, the family version being that he was on his normal commute to work at a bus stop?"

Boston voter registration records for Usaama Rahim list him as a student. Records indicate that as recently as two years ago he was licensed as a security officer in Miami but don’t specify in what capacity.

The Islamic Society of Boston Cultural Center said its security firm hired Rahim as a security guard for a month in mid-2013. Executive director Yusufi Vali said Rahim didn’t regularly pray at the center and didn’t volunteer there or serve in any leadership positions.

On Tuesday afternoon, authorities raided a home in suburban Everett in connection with the case. Everett police confirmed they assisted the FBI in taking a man into custody but said he was taken to Boston, a 10-minute drive away.

Authorities also were searching a home in Warwick, Rhode Island, but wouldn’t confirm that was linked to the Boston shooting.

The officer and the agent involved in the shooting weren’t physically injured but were evaluated at a hospital for what Evans described as "stress."

Vincent Lisi, special agent in charge of the Boston FBI office, said authorities "don’t think there’s any concern for public safety out there right now."

AP writer Philip Marcelo contributed to this report.