The southern US state of Alabama on Thursday put to death a convicted murderer using nitrogen gas, the first time the country has used a method that the UN rights chief said may amount to torture.
Kenneth Eugene Smith was pronounced dead at 8:25 pm (0225 GMT Friday), according to the state attorney general.
"Justice has been served. Tonight, Kenneth Smith was put to death for the heinous act he committed over 35 years ago," the statement by Attorney General Steve Marshall said.
Smith, 58, was on death row for more than three decades after being convicted of the 1988 murder-for-hire of a pastor's wife.
He was put to death at Holman Prison in Atmore, Alabama by nitrogen hypoxia, which involved pumping nitrogen gas into a facemask, causing him to suffocate.
According to media witnesses, he "began writhing and thrashing for approximately two to four minutes, followed by around five minutes of heavy breathing," local news outlet AL.com reported.
Smith appeared to be "holding his breath as long as he could" and there was "involuntary movement" and gasping, Alabama Department of Corrections Commissioner John Hamm told reporters.
The curtain over the media witness room opened at 7:53 pm, AL.com said, with Smith pronounced dead less than 35 minutes later.
The United Nations' human rights chief Volker Turk said he deeply regretted the execution.
"This novel and untested method of suffocation by nitrogen gas may amount to torture, or cruel, inhuman or degrading treatment," he said, according to a statement.
Robin Maher, executive director of the Death Penalty Information Center, called it "an untested, unproven method of execution."
"It's never been used before to execute anyone in the United States, or anyone in the world as far as we know," Maher told AFP.
'Step backward'
Smith was subjected to a botched execution attempt in November 2022, when prison officials were unable to set intravenous lines to administer a lethal injection.
Smith's last words Thursday were, "Tonight, Alabama caused humanity to take a step backward," according to the local CBS affiliate.
"I am leaving with love, peace and light... I love you. Thank you for supporting me. I love all of you," Smith said.
According to the Alabama Department of Corrections, Smith had a last meal of steak, hash browns and eggs on Thursday morning.
The last US execution using gas was in 1999 when a convicted murderer was put to death using hydrogen cyanide gas.
There were 24 executions in the United States in 2023, all of them carried out by lethal injection.
Alabama is one of three US states that have approved the use of nitrogen hypoxia as a method of execution, along with Oklahoma and Mississippi.
'Particularly cruel'
While nitrogen gas had never previously been used to execute humans in the United States, it is sometimes used to kill animals.
But Turk's office pointed out before the execution that even the American Veterinary Medical Association recommends giving large animals a sedative when being euthanized in this manner.
Alabama's protocol for execution by nitrogen asphyxiation makes no provision for sedation.
A spokesman for the European Union, which opposes the death penalty, said the 27-country bloc "deeply regrets" the execution.
"According to leading experts, this method is a particularly cruel and unusual punishment," he said in a statement.
The state of Alabama defended the method as "perhaps the most humane method of execution ever devised."
Smith and an accomplice, John Parker, were convicted of the 1988 murder of Elizabeth Sennett, for which they were each paid $1,000. Parker was executed by lethal injection in 2010.
Charles Sennett, who had arranged his wife's murder, killed himself a week after her death.
Speaking to reporters after the execution Thursday, Elizabeth Sennett's son Mike said it had been a "bittersweet" day for his family, as "nothing that happened here today is going to bring Mom back."
The US Supreme Court rejected Smith's appeal for a stay of execution.
According to a recent Gallup Poll, 53 per cent of Americans support the death penalty for someone convicted of murder, the lowest level since 1972.
Capital punishment has been abolished in 23 US states, while the governors of six others -- Arizona, California, Ohio, Oregon, Pennsylvania and Tennessee -- have put a hold on its use.
"The death penalty is inconsistent with the fundamental right to life," said UN rights chief Turk.
"I urge all states to put in place a moratorium on its use, as a step towards universal abolition."