While officers said they were continuing their investigation, and prosecutors said they have not ruled out the option of bringing charges against anyone, including Baldwin, 63, police said it appeared that the actor had fired a single round from a .45 Lee handgun.
At a news conference on Wednesday, Santa Fe County sheriff Adan Mendoza said the bullet had been passed to investigators by medical officials who removed it from the shoulder of the injured director.
“The actual lead projectile fired was recovered from the shoulder of Mr Souza. The projectile was recovered and medical personnel where he was being treated, turned over to the sheriff's office as evidence,” he said.
“We regard this spent casing and recovered projectile to be the live round that was fired from the revolver by Mr Baldwin.”
Mr Mendoza said what is thought to be additional live rounds were found on the set but they would be subject to testing by ballistics experts.
He said the gun was a Long Colt revolver. “It’s a suspected live round that was fired ... and it did cause injury. That would lead us to believe it was a live round,” he said.
“I won’t comment further on how [the suspected live bullets] got there. This investigation is active, so I won’t comment on how they got there, but we suspect that they are there.”
He said: “That will be determined when testing is done by the crime lab in reference to whether or not they are officially live rounds or not.”
It had previously been reported the actor had been handed the weapon by a member of the crew who had shouted “cold gun”, to indicate the weapon did not contain a live round and was therefore safe to fire.
Mr Mendoza said: “The facts are clear. A weapon was handed to Mr Baldwin. The weapon is functional and fired a live round, killing Ms Hutchins and injuring Mr Souza.”
The sheriff said authorities were now conducting further investigations to determine how crew members obtained a gun with a live round in it.
“The people that inspected or handled the firearm when it was loaded before it got to Mr Baldwin – we’re interviewing. And there [are] some follow-up questions that we need to do,” he said.
Mr Mendoza, and district attorney Mary Carmack-Altwies, in their first press conference about the case, said they had not ruled out the option to bring charges against anyone.
Mr Mendoza said the gun used in the shooting had been handled by two people in addition to Baldwin – the armourer Hannah Gutierrez-Reed and assistant director David Halls.
“The investigation will continue, and if the sheriff’s office determines during our investigation a crime has occurred and probable cause exists, an arrest or arrests will be made and charges will be filed,” said Mr Mendoza.
Asked about the possibility that Baldwin will face charges, Mr Mendoza said “No one has been ruled out ... He was the person who fired the weapon.”
Ms Carmack-Altwies said her office was ready to bring charges if the investigation found sufficient evidence to do so.
“I must emphasise that a complete and thorough investigation is critical to the DA review,” she said. “We take the corroborated facts and evidence and connect it to New Mexico law, and we are not at that juncture yet.”
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