The special prosecutors' decision to drop the charges against Alec Baldwin over the fatal on-set "Rust" shooting was made, at least in part, because investigators found the gun that fired to be mechanically improper, a source familiar with the matter told ABC News.

Baldwin, 65, had been charged with two counts of involuntary manslaughter after fatally shooting cinematographer Halyna Hutchins, 42, on the New Mexico set of the Western in October 2021. The actor was practicing a cross-draw when the gun fired, striking the cinematographer.

Investigators effectively conducted an autopsy of the Colt .45 revolver and found that there were worn joints and that the trigger control was not functioning properly, according to the source.

It became evident to prosecutors the gun could fire without pressure on the trigger, according to the source.

On Thursday, the newly appointed special prosecutors in the case confirmed charges have been dropped against the actor, though noted their investigation remains "active and ongoing."

Baldwin told ABC News' George Stephanopoulos in an exclusive interview in the wake of the shooting that he had "no idea" how a live bullet got onto the set of his film, but that he "didn't pull the trigger" on the firearm that killed Hutchins.

"I would never point a gun at anyone and pull the trigger at them, never," Baldwin told Stephanopoulos.

This is a developing story. Please check back for updates.