FBI testing of the gun used in the fatal shooting on the movie set of "Rust" found that the weapon handled by actor Alec Baldwin could not be fired without pulling the trigger while the gun was cocked, according to a newly released forensics report.

Baldwin had the gun while rehearsing a scene of the Western film at the Bonanza Creek Ranch in New Mexico in October when a shot fired, killing cinematographer Halyna Hutchins and injuring director Joel Souza.

In December, Baldwin told ABC News he never pulled the trigger of the gun that shot Hutchins. "The trigger wasn't pulled. I didn't pull the trigger," Baldwin said.

Baldwin, in that interview, also described cocking the gun as he talked through the scene with Hutchins. "So then I said to her, 'Now in this scene, I'm going to the gun.' And I said, 'Do you want to see that?' And she said, 'Yes.' So I take the gun and I start to cock the gun. I'm not going to pull the trigger."

Cocking a revolver pistol like the one used on the movie set involves pulling the hammer of the gun back to prepare the weapon to fire. When the hammer of the gun is released forward with enough force -- as happens when the trigger is pressed -- it strikes the primer of a round of ammunition causing the gun to fire.

The FBI forensic report was handed over to the Santa Fe County Sheriff's office as part of the ongoing investigation into the fatal on-set shooting.

The report found that the gun, a .45 Colt (.45 Long Colt) caliber F.lli Pietta single-action revolver, "could not be made to fire without a pull of the trigger" with the hammer cocked at the 1/4 and 1/2 positions. It also found that when the weapon was fully-cocked it "could not be made to fire without a pull of the trigger while the working internal components were intact and functional."

FBI examiners observed an internal malfunction of the gun during testing at the fully-cocked position, with the report noting "portions of the trigger sear and cylinder stop fractured while the hammer was struck."

The FBI report noted the limitations of the forensics testing, saying "it may not be possible to recreate or duplicate all of the circumstances which led to the discharge of a firearm without a pull of the trigger."

CNN has reached out to a representative for Baldwin for comment.

An attorney representing Hannah Gutierrez Reed, who served as the armorer and props assistant on the film, said the forensics report indicated "Baldwin had to have pulled the trigger to fire the revolver" and that the 24-year-old was being used as a "scapegoat."

Part of the Santa Fe County police investigation focuses on how a live round of ammunition may have made it onto the movie set.

In April, Rust Movie Productions, LLC was fined nearly US$137,000 and cited for having a culture of "plain indifference to employee safety" on set, according to a report from New Mexico Environment Department's Occupational Health & Safety Bureau.