A US judge has agreed with lawyers for actor Alec Baldwin that prosecutors and police withheld evidence in his trial on involuntary manslaughter charges over the death of cinematographer Halyna Hutchins in 2021.

Three days after Baldwin’s trial began, New Mexico district court Judge Mary Marlowe Sommer threw out the case as the actors’ lawyers alleged a “cover up” by prosecutors who’ve been plagued by missteps since first filing charges 18 months ago.

Breaking down in tears, the multiple Emmy-award winning actor hugged his wife Hilaria Baldwin as other family members wept in the public gallery.

Baldwin faced an unprecedented criminal prosecution as an actor for an on-set shooting and his lawyers said prosecutors dragged him through a “cesspool of improprieties.” Baldwin and his family left court without speaking to reporters.

“The state’s withholding of the evidence was wilful and deliberate,” Sommer said in delivering her decision. “Dismissal with prejudice is warranted to ensure the integrity of the judicial system and the efficient administration of justice.”

The actor’s lawyer Alex Spiro told the court that the Santa Fe sheriff’s office took possession of live rounds in March as evidence in the case but failed to list them in the Rust investigation file or disclose their existence to defence lawyers.

“The real reason you didn’t inventory that evidence is because it could have jeopardised the law enforcement case,” Spiro told Santa Fe County Sheriff’s Office Corporal Alexandria Hancock, the lead investigator on the Rust case, in cross examination.

Erlinda Johnson, one of the state prosecutors on the case, resigned on Friday, the fourth prosecutor to quit or be forced to step down.

“I did not intend to mislead the court,” lead state prosecutor Kari Morrissey said after taking the unusual step of defending herself from the witness stand.

“My understanding of what was dropped off at the sheriff’s office is on this computer screen and it looks absolutely nothing like the live rounds from the set of Rust.”

Hutchins died in Hollywood’s first on-set shooting in nearly 30 years when Baldwin was directed to point a revolver at her as she set up a camera shot during filming southwest of Santa Fe.

The weapon fired a .45 calibre round inadvertently loaded by the movie’s armourer Hannah Gutierrez.

The Colt .45 rounds at the centre of the dismissal were handed into the sheriff’s office on March 6 by Troy Teske, a friend of Thell Reed, the stepfather of Gutierrez, on the same day she was convicted of involuntary manslaughter for Hutchins’ death.

A Sheriff’s office crime scene technician testified on Thursday that the rounds did not match those collected on the set of Rust which were sent for FBI testing.

“That turned out to be completely false, didn’t it?” Baldwin’s lawyer Spiro asked Corporal Hancock.

“You’re correct,” she said.

Judge Sommer asked Hancock who had decided to put Teske’s ammunition into a separate case file number.

Hancock said it was the decision of her supervisor, prosecutors and herself.

“Ms. Morrissey was part of that conversation?” asked Sommer, growing visibly angry.

“Yes,” replied Hancock.

Spiro also questioned Morrissey about her attitude toward his client, saying witnesses had reported she had characterised him with expletives and said she would try to teach him a lesson.

“I never said to witnesses that I would teach him a lesson,” she said.

Prosecutors had alleged Baldwin played a role in the death of Hutchins because he handled the gun irresponsibly.

His lawyers said Baldwin was failed by Gutierrez and others responsible for safety on the set, and law enforcement agents were more interested in prosecuting their client than finding the source of the live round that killed Hutchins.

Defence lawyers alleged prop supplier Seth Kenney supplied the live rounds to Rust, an accusation he denied in testimony on Friday.

It remains to be seen whether the dismissal of Baldwin’s case would affect Gutierrez’s conviction, which is under appeal.

—AAP