Santa Fe, New Mexico - Alec Baldwin's trial for involuntary manslaughter collapsed sensationally Friday as a judge dismissed the case after finding that key evidence over the fatal shooting on the set of Rust had been withheld from the defense.

Judge Mary Marlowe Sommer, presiding over the trial in Santa Fe, New Mexico, said bullets potentially linked to the death of cinematographer Halyna Hutchins that could have been favorable to Baldwin's case had not been shared with his lawyers by police and prosecutors.

The Hollywood star immediately burst into tears, as the case – which could have seen him sentenced to 18 months in prison if found guilty – was abruptly tossed out in sensational scenes.

"The state's willful withholding of this information was intentional and deliberate. If this conduct does not rise to the level of bad faith, it certainly comes so near to bad faith as to show signs of scorching prejudice," Marlowe Sommer said.

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"The court concludes that this conduct is highly prejudicial to the defendant."

Baldwin was holding a gun in the direction of Hutchins during a rehearsal in October 2021 when the weapon fired, killing Hutchins and wounding the film's director.

The movie's armorer Hannah Gutierrez, who loaded the fatal weapon, is already serving 18 months in prison for involuntary manslaughter.

Baldwin was facing the same charge. Prosecutors claim he ignored basic gun safety laws and acted recklessly on set.

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Baldwin's celebrity lawyer Alex Spiro argued the actor had no responsibility for checking the weapon's deadly contents and did not know it contained live rounds.

But the defense's case also rested heavily on discrediting the police investigation.

And Spiro on Thursday introduced evidence that live bullets potentially linked to the shooting had been handed to police, but not disclosed to Baldwin's lawyers.

The bullets were handed into police by a "Good Samaritan" earlier this year, more than two years after the Rust tragedy. The "Good Samaritan" was a former police officer and a family friend of Gutierrez, the armorer. He told police the bullets matched the rounds that killed Hutchins.

Spiro accused police of having "buried" evidence by not filing it under the Rust case, depriving the defense of a chance to see it.

"Which was a perfect plan," he told the court, which led to Judge Marlowe Sommer initially pausing the trial.

Crime scene technician Marissa Poppell, under questioning from Spiro, said she had cataloged the bullets, but had been told not to file them under the Rust case.

Special prosecutor Kari Morrissey scrambled to respond to the development, telling the court she had never seen or heard of the bullets before this week.

But when it emerged that Morrissey had been present for discussions in which it was decided not to submit the bullets into the Rust case file, Morrissey voluntarily called herself as a witness in a last-ditch attempt to salvage the state's case.

She claimed the allegedly "buried" bullets did not match the live rounds on the set of Rust, and had been stored in a different state, Arizona, up to the day of the tragic shooting.

Marlowe Sommer was not convinced and dismissed the case, prompting the emotional scenes among Baldwin and his family.

The withheld evidence could also benefit Gutierrez, who has already launched an appeal, and David Halls, the movie's first assistant director, who took a plea deal and admitted negligence.

Christopher Melcher, a Los Angeles-based attorney, told AFP he was "dumbfounded" by the case's sudden collapse.

"The case unfolded as quickly as I've ever seen a case... It was just absolutely amazing to watch."