SANTA FE, N.M. (AP) — The scheduled trial next week of a movie weapons supervisor in the fatal shooting of a cinematographer by Alec Baldwin may hinge on an enduring mystery: How did live ammunition find its way onto the set of a film set where it was expressly prohibited?

Investigators recovered six live rounds of ammunition from a box, a bandolier, a gun belt and other locations on the set of the Western movie “Rust,” including the round that killed cinematographer Halyna Hutchins and wounded director Joel Souza.

Special prosecutors say they’ll present “substantial evidence” at the trial that movie armorer Hannah Gutierrez-Reed unwittingly brought live rounds onto the set when she first began work on the film.

They say that includes photos showing that live rounds were present on the set days before Hutchins was killed. They also plan to present testimony that, months prior to the shooting, Gutierrez-Reed had looked for and purchased live .45-caliber ammunition.

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“Ms. Gutierrez is not charged with intentional homicide, she is charged with homicide based on negligence,” special prosecutor Kari Morrissey said in a recent court filing. “The tragedy occurred due to a series of negligent acts given that live rounds were on set well before October 21, 2021. Her ongoing negligent acts created numerous opportunities for live rounds to go undetected.”

Gutierrez-Reed has pleaded not guilty to the involuntary manslaughter charge.

Flimsy is how her attorneys describe the evidence that she might have unknowingly brought live rounds on set, saying it falls far short of standards for prosecution.

Her attorneys also accuse prosecutors of compromising a crucial trial witness by handing over privileged communications about their case to the Albuquerque-based dummy ammunition supplier for “Rust” — whom they contend is the source of live ammunition that made its way onto the set. A civil lawsuit by Gutierrez-Reed against ammunition supplier Seth Kenney was dismissed in August and can’t be refiled.

Much of the evidence about ammunition on set — culled from sources including thousands of text messages between “Rust” crew members — has not been made public under commonplace rules of evidentiary discovery prior to trial.

The proceedings against the armorer hold implications for Baldwin, the lead actor and co-producer on “Rust.” He has pleaded not guilty to a charge of involuntary manslaughter and could face trial later this year. “Rust” assistant director and safety coordinator David Halls pleaded no contest to unsafe handling of a firearm and received a suspended sentence of six months of probation, agreeing to cooperate in investigations of the shooting.

Prosecutors allege that Gutierrez-Reed eventually loaded a live round into the gun that Baldwin discharged during the October 2021 rehearsal, killing Hutchins, and that the tragedy was a consequence of lax oversight of ammunition.

Baldwin has said he assumed the gun had only inert dummy rounds inside that can’t fire and that someone else is responsible.

But the indictment against Baldwin provides two alternative standards for prosecution, one based on the negligent use of a firearm and another tied to negligence without due caution or “circumspection,” also defined as “total disregard or indifference for the safety of others.” Legal experts say the latter standard could broaden the investigation beyond Baldwin’s handling of the gun. A date has not been set for Baldwin’s potential trial.

“Rust” used an operable revolver. Industry-wide guidance, under a bulletin that applied to “Rust,” says that “live ammunition is never to be used nor brought onto any studio lot or stage.” It also says to “treat all firearms as if they are loaded.”

Crew members also say Bonanza Creek Ranch, the movie set location where Hutchins was shot, forbade the presence of live ammunition on its property.

State workplace safety regulators say Gutierrez-Reed was responsible for storage, maintenance and handling of firearms and ammunition on set and for loading firearms with blanks that have a charge but no projectile, or inert dummy rounds.

Live rounds are typically distinguished from dummy rounds by a small hole in the dummy’s brass cartridge, indicating there is no explosive inside, by a missing or dimpled primer at the bottom of the cartridge, or by shaking the round to hear the clatter of a BB that is inserted inside.

Live ammunition has made its way onto U.S. movie sets with severe consequences in just a handful of instances.

Actor Brandon Lee died in 1993 after he was shot in the abdomen while filming a scene of “The Crow.” Lee was killed by a makeshift bullet that remained in a gun from a previous scene. The production ended up paying a $55,000 fine to federal regulators.

In 2005, federal regulators fined Greystone Television and Films $650 after a crew member was shot in the thigh, elbow and hand. It turned out that balloon-breaking birdshot rounds were in the same box as the blanks that were supposed to be used in rifles.

In New Mexico, a scathing report from state regulators about the “Rust” shooting said the production company did not develop a process for ensuring live rounds were kept away from set and failed to give the armorer enough time to thoroughly inventory ammunition.

Prosecutors want the regulators’ conclusions kept out of the trial because it might be used argue that “Rust” management was responsible for safety failures and not Gutierrez-Reed.

Heated and disparaging exchanges between defense counsel and prosecutors in recent pretrial court filings include accusations of “vindictive,” unconstitutional prosecution tactics. Special prosecutors Kari Morrissey and Jason Lewis are pursuing additional felony charges of tampering with evidence on allegations that Gutierrez-Reed handed off a baggie of possible narcotics to another crew member in the aftermath of the shooting to evade prosecution and took a video of herself bringing a gun into a Santa Fe bar weeks before the fatal shooting.

Defense attorney Jason Bowles says prosecutors are using trumped-up charges to pressure Gutierrez-Reed to make a false confession regarding the source of live ammunition on the film set.

“The state has always been open to resolving Ms. Gutierrez’s cases,” special prosecutor Morrissey responded in a court filing, “on one condition — that she take responsibility for the fact that she unknowingly brought live ammunition onto the set of ‘Rust.’”