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 Centre.
Alabama officials had said in an earlier court filing they expected Smith to lose consciousness within seconds and die in a matter of minutes.
Meanwhile, state officials said the execution went as planned.
Commissioner of the Alabama Department of Corrections,John Hamm, 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,” Mr Hamm 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.
“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.
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.
BBC