Alec Baldwin has informed a prosecutor and sheriff in the US state of New Mexico that he may sue them after it emerged they withheld evidence from his defence team during their attempt to prosecute him over a fatal shooting on the Rust film set.

A New Mexico judge dismissed involuntary manslaughter charges against the actor on the third day of trial on Friday, agreeing with his lawyers that prosecutor Kari Morrissey and the sheriff's office concealed evidence about the source of the live round that killed the movie's cinematographer Halyna Hutchins in 2021.

Mr Baldwin's lawyers sent letters to Ms Morrissey and Santa Fe County Sheriff Adan Mendoza asking them to preserve documents for future litigation, according to copies of the letters seen by Reuters.

The Santa Fe County Sheriff's Office confirmed it received the letter while Ms Morrissey did not respond to a request for comment.

Ms Hutchins died when Mr Baldwin pointed a gun at her as they set up a camera shot on the film set near Santa Fe.

The gun fired a live round inadvertently loaded by chief weapons handler Hannah Gutierrez who was convicted of involuntary manslaughter in March.

Hannah Gutierrez was convicted of involuntary manslaughter

Ms Gutierrez is serving an 18-month prison sentence and has filed a motion for dismissal of the charges due to the failure of prosecutors to disclose evidence on the live rounds, testing of Mr Baldwin's gun by firearms expert Lucien Haag and an interview with the film's props supplier Seth Kenney.

Erlinda Johnson resigned as Ms Morrissey's assistant prosecuting the case on Friday, shortly before Judge Mary Marlowe Sommer dismissed charges against Mr Baldwin.

Ms Johnson said that, since she was appointed in April, she gave the defence unredacted documents after they got in touch with her seeking information when Ms Morrissey did not provide documents they requested.

"As prosecutors we have obligations to disclose all the evidence," Ms Johnson said, adding that she believed it was right to dismiss the case.

"We not only owe a duty to the people, but to the defendants accused of crimes as well."

Ms Johnson said she did not give Mr Baldwin's legal team details of the live rounds and only learned of that evidence when the defence did during testimony in court.