Hollywood star Alec Baldwin said Friday his "heart is broken" after he fatally shot a cinematographer with a prop gun he had been told was safe on the set of a Western about an accidental killing.

The US actor and producer said he was "fully cooperating" with a police investigation, as law enforcement officials carried out a search warrant, and reports emerged of allegedly dangerous working conditions on the set.

"There are no words to convey my shock and sadness regarding the tragic accident that took the life of Halyna Hutchins, a wife, mother and deeply admired colleague of ours," Baldwin tweeted.

Baldwin had been handed a prop gun by an assistant director who did not know it was loaded and shouted "Cold gun" -- meaning it contained no live ammunition -- according to a search warrant seen by the Santa Fe Reporter.

Performing a rehearsal scene, Baldwin fired the gun, striking Hutchins in the chest, and the film's director in the shoulder. Hutchins, 42, was transported to hospital by helicopter but died of her wounds.

"My heart is broken for her husband, their son, and all who knew and loved Halyna," wrote Baldwin.

Alec Baldwin
Alec Baldwin is co-producer on "Rust," a 19th century western about the bond between a grandfather and his grandson after the boy is convicted of murder. GETTY IMAGES NORTH AMERICA / MATTHEW STOCKMAN

Production of "Rust" -- a 19th-century Western in which Baldwin is playing the lead -- has been suspended. Souza, the 48-year-old director, was taken by ambulance to hospital and later released.

Santa Fe sheriff spokesman Juan Rios told AFP that Baldwin came in voluntarily and "provided statements and answered their questions," but no charges have been filed and no arrests made.

He confirmed that investigators were executing a search warrant at the site of the shooting.

The warrant allows investigators to gather camera footage, cellphones, weapons and ammunition, and clothing worn by Baldwin and others which may have been splattered with blood, according the affidavit.

Meanwhile the Los Angeles Times reported that around six crew members had walked off set before the shooting to protest working conditions, including long hours. It said Hutchins was among those who had called for safer working conditions.

There had been at least three prop gun misfires prior to the fatal accident, the LA Times report said, citing anonymous crew members who were on set.

Halyna Hutchins was transported to hospital by helicopter but died of her wounds
Halyna Hutchins was transported to hospital by helicopter but died of her wounds GETTY IMAGES NORTH AMERICA / Sonia Recchia

The production company did not respond to an AFP request for comment.

Los Angeles-based Hutchins, originally from Ukraine, was selected as one of American Cinematographer's Rising Stars of 2019, according to her website.

Ukraine's foreign ministry said its consulate general in San Francisco was working with US authorities and speaking to Hutchins' relatives.

Hutchins' union IATSE -- which represents film industry technicians including cinematographers -- said it was "heartbroken and devastated to learn that one of our members, sister Halyna Hutchins, died yesterday."

'Rust' director Joel Souza was transported to Christus St. Vincent Medical Center
'Rust' director Joel Souza was transported to Christus St. Vincent Medical Center GETTY IMAGES NORTH AMERICA / Sam Wasson

"Creating a culture of safety requires relentless vigilance from every one of us, day in and day out," it added.

The union's branch representing prop masters sent an email to members stating that "a live single round was accidentally fired," although law enforcement has yet to confirm details about the firearm involved.

Movie sets usually have stringent rules over the use of prop weapons, but accidents have happened.

Most famously, Brandon Lee, the son of martial arts legend Bruce Lee, died during filming of "The Crow" after being shot by a gun that was supposed to fire blanks.

Brandon's sister Shannon Lee wrote on Twitter: "No one should ever be killed by a gun on a film set."

"It's time to stop being macho about blanks and end the practice," tweeted director Ben Rock, noting that gunshot effects such as the loud bang and muzzle flash can be added in post-production with modern technology.

But "the problem with dummy weapons is that you take away the recoil and the smoke, which adds something to the acting," said Hollywood armorer Guillaume Delouche.

Meanwhile, tributes to Hutchins poured in on social media.

"My greatest fear is that someone will be fatally hurt on one of my sets," tweeted Guardians of the Galaxy director James Gunn.

"My heart goes out to all of those affected by the tragedy today on Rust, especially Halyna Hutchins & her family."

The shooting took place at the Bonanza Creek Ranch, a production location near Santa Fe which is popular with Hollywood filmmakers.

Baldwin co-produced the film and stars as Harland Rust, an outlaw whose grandson is convicted of murder, and who goes on the run with him.

A leading man on television and in films since the 1980s, Baldwin has starred in high-profile movies including "The Hunt for Red October."

He garnered new fans for TV comedy "30 Rock" and with his long-running portrayal of Donald Trump on "Saturday Night Live," a character that irritated the former president.

Baldwin also made headlines for his marriage -- and acrimonious divorce -- to Kim Basinger, his outspoken political views and hot temper.

He lives near New York with second wife Hilaria. The couple have six children together.