Alabama executed convicted murderer Kenneth Smith by asphyxiating him with nitrogen gas on Thursday, the first use of a new method of capital punishment that the state is advancing as a simpler alternative to lethal injections.

The state has called its new protocol "the most painless and humane method of execution known to man."

United Nations human rights experts and lawyers for Smith had sought to prevent it, saying the method was risky, experimental and could lead to a torturous death or non-fatal injury.

Smith, convicted of a 1988 murder-for-hire, was a rare prisoner who had already survived one execution attempt. In November 2022, Alabama officials aborted his execution by lethal injection after struggling for hours to insert an intravenous line's needle in his body.

In Smith's second and final trip to the execution chamber on Thursday, executioners restrained him in a gurney and strapped a commercial industrial-safety respirator mask to his face. A canister of pure nitrogen was attached to the mask that once flowing, deprived him of oxygen.

The execution began at 7:53 p.m. (0153 GMT Friday) and Smith was declared dead at 8:25 p.m. (0225 GMT), prison officials said.

Smith appeared to remain conscious for several minutes after the nitrogen was activated, according to five journalists were allowed to watch the execution through glass as media witnesses. He then began shaking and writhing on the gurney for about two minutes, and then could be seen breathing deeply for several minutes before his breathing slowed and became imperceptible, the witnesses said.

Alabama officials had said in court filings they expected Smith would be rendered unconscious in under a minute and die shortly after.

"It appeared that Smith was holding his breath as long as he could," Alabama Corrections Commissioner John Hamm said when asked at a press conference if the writhing had been expected. "He struggled against the restraints a little bit but it's an involuntary movement and some agonal breathing. So that was all expected."

Before the nitrogen was switched on, Smith made a lengthy final statement that began: "Tonight, Alabama caused humanity to take a step backward."

His wife and other relatives attended and he gestured towards them. "I'm leaving with love, peace and light," he said, according to media witnesses. "Love all of you."

Failed challenges

Smith mounted legal challenges in federal courts arguing that Alabama's method amounted to unconstitutional "cruel and unusual punishment," but he failed to cross the high bar needed to have a judge order a delay of his execution.

Smith's lawyers had told courts they feared the mask would not properly seal against Smith's face, allowing oxygen to seep in, delaying or even averting the moment of unconsciousness but risking serious brain injury. They proposed Alabama instead use a hood pre-filled with pure nitrogen, to be plunged over his head, or else to use a firing squad.

Smith's lawyers also told courts Smith had been repeatedly vomiting as his return to the execution chamber drew near. They raised fears he could be sick after the mask was strapped on and choke on his own vomit.

In response, prison officials said they would serve him his final meal on Thursday morning and forbid any solid foods after 10 a.m..

His final meal was steak, hash browns and eggs, prison officials said.

The U.S. Supreme Court's conservative majority rejected Smith's final attempt to have his execution delayed to allow his legal challenge to continue on Thursday evening, and the execution began soon after.

The court did not explain its reasoning in denying Smith's appeal, but the three liberal justices offered written dissents.

"Having failed to kill Smith on its first attempt, Alabama has selected him as its 'guinea pig' to test a method of execution never attempted before," Justice Sonia Sotomayor wrote, saying she would have granted the injunction. "The world is watching."

Approved but unused in other states

U.S. states that use capital punishment have found it increasingly difficult to get drugs for lethal injections, partly because pharmaceutical companies forbid supplying them to prisons to comply with a European trade ban on goods to be used in torture or executions.

Lawmakers in Oklahoma and Mississippi have also approved similar nitrogen-asphyxiation execution protocols in recent years, but have yet to put them into practice.

Smith was convicted of murdering Elizabeth Sennett, a preacher's wife, after he and an accomplice each accepted a $1,000 fee from her husband to kill her, according to trial testimony.

Eleven of 12 jurors voted to sentence Smith to life in prison, but an Alabama judge overruled their recommendation under a law that has since been abolished as unconstitutional.

Several of Sennett's relatives attended the execution and they addressed the media after, saying they had forgiven Sennett's killers.

"Nothing that happened here today is going to bring mom back," Mike Sennett said. "It's a bittersweet day, we're not going to be jumping around, hooping and hollering, hooraying and all that, that's not us. We're glad this day is over."