A .45-caliber Colt revolver used on the set of the film ‘Rust’ was not thoroughly checked before being given to actor Alec Baldwin, who fired a live lead bullet in an accidental fatal shooting last week in New Mexico, according to officials and a new court filing.

New details about the incident emerged on Wednesday during a news conference by Santa Fe County Sheriff Adan Mendoza and District Attorney Mary Carmack-Altwies and in an affidavit filed by the sheriff's department.

Mendoza told reporters there was...