A grainy video with raindrops on the lens shows the fatal standoff between law enforcement and Usaama Rahim, the man who brought a knife to a gunfight one week ago at a Roslindale bus stop.

Suffolk County District Attorney Dan Conley, Boston Police Commissioner Bill Evans and FBI Agent Vincent Lisi showed the surveillance footage captured from a Burger King some 50 yards away from the shooting on Monday morning.

One Boston Police Detective and five FBI Agents, weapons not yet drawn, in plain clothes converge on Rahim as he stood at a bus stop. The law enforcement officials are seen backing away from Rahim.

It is here that authorities say Rahim was instructed to put down a knife he was wielding. A police cruiser is seen turning on his lights on Washington Street. The BPD Detective and an FBI agent opened fire, fatally striking Rahim in the shoulder, upper chest and torso. A school bus drives passed the scene as the bullets fly. Conley said that the officer used digression knowing there was a bus in the backdrop before firing.

“This video is simply one piece of evidence among many, and the investigation is still very active,” Conley said. “I haven’t made any findings yet or come to any legal conclusions. This is an exacting process and it requires a careful analysis of all the evidence in light of state and federal case law. A great deal of work still needs to be done, and this process is too important to rush.”

Conley said that his office acted independently in reviewing the footage and has not come to any conclusions in the exacting process of determining the use of deadly force. The FBI and BPD are conducting their own investigations as well.

Rahim, 26, is said to have adopted radical Islamic beliefs and sought to behead a member of “the boys in blue” with a Marine Bowie Knife. He was under 24/7 surveillance from a joint terrorism task force.

“Nobody could have anticipated what happened that morning,” Evans said. “This guy had a malicious intent. I think we averted a serious tragedy that day.”

Evans said that non-lethal force was not an option even though Rahim was outnumbered. Law enforcement officials approached Rahim without a warrant to talk to him in what is called a Terry Stop. But Evans said that knowing his intent set the tone of the events that followed.

“Mr. Rahim had been under 24-hour surveillance by the FBI for some time,” Lisi said. “Last Tuesday, we intercepted communication that indicated that he was going to carry out an attack that day.”

“Taking a life is the hardest thing we have to do and we have to live with that decision,” Evans said. “It was us or him in this situation.”

Rahim was allegedly in contact with his nephew, David Wright of Everett, who was arrested the evening of the shooting and was charged with conspiracy to obstruct justice. He'd joked in wire-tapped phone conversations with Wright about “thinking with your head on your chest,” which FBI Special Agent Joseph Galietta described as “a reference of some foreign terrorist organizations to behead targets and place their heads on their chests in propaganda videos” in an affidavit issued in federal court.

Rahim was buried in Canton last Friday.