Updated

Prosecutors will not seek criminal charges against a Boston officer and an FBI agent who fatally shot a man last year who they believed was plotting an attack against police, authorities announced on Wednesday.

"The use of deadly force was a lawful exercise of self-defense," Suffolk District Attorney Daniel Conley said at a press conference.

Usaamah Rahim, 26, was fatally shot June 2, 2015, in the city's Roslindale neighborhood. Rahim had been under 24-hour surveillance for several months as part of an investigation into his ties with ISIS, including a plot with two co-conspirators to behead a political activist in New York City, according to authorities.

The morning of the shooting, a recorded phone conversation revealed that Rahim had abandoned his plans to travel to New York and was instead planning to attack officers in Boston, Conley said.

"It was clear from this recorded conversation that Mr. Rahim did not expect to survive this attack," he said.

Terror investigators approached Rahim at a bus stop, at which point Rahim drew a large, military-style knife from a sheath, Conley said.

Conley's office reviewed dozens of witness statements and police reports and footage from surveillance cameras. The evidence proves "beyond a reasonable doubt" that Rahim was armed with a knife and "posed the threat of death," Conley said.

Rahim was shot three times and taken to a nearby hospital, where he was pronounced dead.

Nicholas Rovinski, of Warwick, Rhode Island, and David Wright, of Everett, Massachusetts, are facing federal charges they plotted with Rahim to kill civilians in support of ISIS. They have pleaded not guilty.

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