US actor Alec Baldwin faces involuntary manslaughter charges carrying prison time for the fatal shooting of cinematographer Halyna Hutchins during the 2021 filming of the Rust movie in New Mexico.
Santa Fe's top prosecutor Mary Carmack-Altwies said on Thursday the film's armourer, Hannah Gutierrez-Reed, would also be charged with involuntary manslaughter.
She said charges would be filed by the end of this month.
Hutchins' family welcomed the announcement while Baldwin's lawyer Luke Nikas said "this decision distorts Halyna Hutchins' tragic death and represents a terrible miscarriage of justice".
The charging decision followed more than a year of investigation after a pistol was fired in October 2021 as Baldwin, 64, was rehearsing with it on the set of his movie Rust outside Santa Fe.
The movie's director, Joel Souza, was hit and wounded by the same .45-calibre bullet that killed Hutchins.
According to a police report, David Halls, the assistant director who handed the gun to Baldwin, told the actor it was "cold" - an industry term meaning it did not contain live rounds.
Halls signed a plea agreement for the charge of the negligent use of a deadly weapon, Carmack-Altwies said in her statement.
Gutierrez-Reed had handled the gun before Halls.
Prosecutors told the New York Times it was part of industry standards for actors to check guns they used were safe to handle and they should never point them at anyone.
Baldwin has pushed back at that idea, saying firearm safety was the responsibility of the armourer, first director and others.
"You should not point a gun at someone that you're not willing to shoot," Carmack-Altwies said in an interview with the New York Times on Thursday.
But prosecutors could face long odds securing a conviction, according to legal experts, who said if Baldwin was told the gun did not contain live ammunition by on-set professionals, he would not be obligated to inspect it himself.
"It's a very aggressive charging decision and the defence has a strong case," said personal injury attorney and former prosecutor Neama Rahmani, who was not involved in the Rust case.
"Accidents like this are not enough for criminal liability."
Baldwin and Gutierrez-Reed will be charged "in the alternative" with two counts of manslaughter, meaning a jury will decide not only if they were guilty but under which definition of involuntary manslaughter, the prosecutor said.
Simple involuntary manslaughter for negligence is punishable by up to 18 months in jail and a $US5000 ($A7232) fine.
Should prosecutors prove there was more than simple negligence involved in the use of a firearm, they could face a mandatory five-year jail term.
Baldwin has denied responsibility for Hutchins' death and said he did not pull the trigger of the replica Pietta .45-calibre long Colt revolver.
An FBI forensic test of the revolver found it "functioned normally" and would not fire without the trigger being pulled.
"M. Baldwin had no reason to believe there was a live bullet in the gun - or anywhere on the movie set," his lawyer Nikas said.
"He relied on the professionals with whom he worked, who assured him the gun did not have live rounds.
"We will fight these charges, and we will win."
Brian Panish, a lawyer representing the Hutchins family, said his own investigation found charges were warranted.
The family sued Baldwin in 2021, alleging he had a responsibility to check the gun did not contain live rounds, not point it at the cinematographer and not cock the weapon and pull the trigger.
The family subsequently reached a settlement in which Matt Hutchins became an executive producer of Rust.
Production of the film was set to resume early in 2023 outside New Mexico.