The UN, EU and anti-death penalty activists have condemned the first execution of a prisoner using nitrogen gas.
Kenneth Eugene Smith was put to death Thursday night at a state prison in Atmore, Alabama.
A journalist who witnessed the execution told the BBC that Smith thrashed violently on the gurney.
Smith was convicted in 1989 of murdering a preacher’s wife, Elizabeth Sennett, in a killing-for-hire.
The execution process began at 19:53 local time (01:53 GMT) and Smith was pronounced dead about half of an hour later at 20:25 (02:35 GMT).
The UN High Commissioner for Human Rights, Volker Turk, said he had “serious concerns this novel and untested method of suffocation by nitrogen gas may amount to torture, or cruel, inhuman or degrading treatment”.
In a statement, EU officials called nitrogen gas “a particularly cruel and unusual punishment”.
One of the five reporters who witnessed the execution told the BBC it was unlike any other he’d seen.
“I’ve been to four previous executions and I’ve never seen a condemned inmate thrash in the way that Kenneth Smith reacted to the nitrogen gas,”, Alabama journalist Lee Hedgepeth told the BBC’s Newsday programme.
“Kenny just began to gasp for air repeatedly and the execution took about 25 minutes total.”
Inhaling pure nitrogen gas cuts off the oxygen supply to the brain. The procedure had never been used before in an execution, according to the Death Penalty Information Center.
Alabama officials had said in an earlier court filing they expected Smith to lose consciousness within seconds and die in a matter of minutes.
Smith’s spiritual adviser, the Reverend Jeff Hood, said after the execution: “I think that anybody that witnessed this knows that we didn’t see someone go unconscious in two or three seconds.”
“What we saw was minutes of someone struggling for their life,” Mr Hood said. “We also saw cracks in correction officials in the room who were visibly surprised at how bad this thing went.”
State officials, meanwhile, said the execution went as planned.
John Hamm, commissioner of the Alabama Department of Corrections, said it “appeared that Smith was holding his breath as long as he could.”
“He struggled against restraints a little bit, but there’s some involuntary movement and some abnormal breathing,” he told a post-execution briefing. “That was all expected and was in the side effects that we’ve seen or researched on nitrogen hypoxia”.
“Nothing was out of the ordinary,” he added.
At a press conference on Friday, Alabama Attorney General Steve Marshall said he expected the method to be used in the future and by other states. Oklahoma and Mississippi have also authorised nitrogen gas for executions.
“Despite the international effort by activists to undermine and disparage our state’s justice system and to deny justice to the victims of heinous murders, our proven method offers a blueprint for other states and a warning to those who would contemplate shedding innocent blood,” he said in a statement after the execution on Thursday.
White House Press Secretary Karine Jean-Pierre told reporters on Friday that the Biden administration found the execution “very troubling” and that it would continue to enforce a moratorium on executions to be carried out by the federal government.
Smith’s last words were, according to a media pool report: “Tonight Alabama caused humanity to take a step backward. I’m leaving with love, peace and light. Thank you for supporting me. Love all of you.”
He also made an “I love you” sign with sign language, reporters said.
Smith was one of two men convicted of murdering Mrs Sennett.
Her husband Charles Sennett, a debt-ridden preacher, paid them $1,000 (£790) to carry out the murder so that he could collect insurance money. He killed himself as investigators closed in.
Mrs Sennett was beaten with a fireplace tool and stabbed in the chest and neck, and her death was staged to look like a home invasion and burglary.
At his trial Smith admitted to being present when she was killed, but said he did not take part in the attack.
Smith’s fellow hitman, John Forrest Parker, was executed in 2010.
Mike Sennett, the victim’s son, said the family wouldn’t be celebrating but that Smith had paid his “debt”.
“Nothing that happened here today is going to bring Mom back,” he said. “We’re glad this day is over.”
In a statement, Smith’s legal team said it was “deeply saddened” by his execution, noting the jury in his case had voted to spare his life but a judge overrode its decision.
American opponents of the death penalty also condemned the execution.
“It’s not something that should be done, and yet it was done,” said Abraham Bonowitz, executive director of Death Penalty Action.
Alabama tried to execute Smith by lethal injection two years ago, but prison staff had trouble inserting an intravenous line.
On Thursday night, the US Supreme Court denied him a last-minute reprieve. Three liberal justices dissented from the conservative majority’s ruling.
The death penalty is on decline in the US.
Last year 24 people were put to death with lethal injections in five states: Texas, Florida, Missouri, Oklahoma and Alabama.
A slight majority – 53% – of Americans approve of the death penalty in murder cases, according to a long-running Gallup poll.