Alabama will attempt to put an inmate to death with nitrogen gas on Thursday night, a never-before-used execution method that the state claims will be humane but critics call cruel and experimental.

Kenneth Eugene Smith, a 58-year-old convicted killer whose 2022 lethal injection was called off at the last minute because authorities could not connect an IV line, is scheduled to be executed at an Alabama prison.

Alabama plans to put an industrial-type respirator mask over Smith’s face and replace his breathing air with pure nitrogen gas, causing him to die from lack of oxygen.

The execution will be the first attempt to use a new execution method since the 1982 introduction of lethal injection, now the most common execution method in the United States.

Attorneys for Smith have waged a legal battle to halt the execution, arguing that the state is seeking to make Smith the “test case” for the new execution method, which merits more legal scrutiny before it is used on an inmate.

“It’s an experiment,” said the Rev Jeff Hood, Smith’s spiritual advisor and a death penalty opponent.

The US Supreme Court on Wednesday rejected Smith’s argument that it would be unconstitutional to make another attempt to execute him after the failed lethal injection.

The US 11th Circuit Court of Appeals also declined to halt the execution on Wednesday, saying Smith had not sufficiently supported claims that the new execution method would violate the constitutional ban on cruel and unusual punishment.

His attorneys are expected to appeal that decision to the US Supreme Court.

Smith is one of two men convicted in the 1988 murder-for-hire slaying of Elizabeth Sennett.

Prosecutors said he and the other man were each paid 1,000 dollars to kill Sennett on behalf of her pastor husband, who was deeply in debt and wanted to collect on insurance.

Alabama plans to strap Smith to a gurney in the execution chamber — the same chamber where he was strapped down for several hours during the lethal injection attempt — and place a “full facepiece supplied air respirator” over his face.

After he is given a chance to make a final statement, the warden, from another room, will activate the nitrogen gas.

The nitrogen will be administered through the mask for at least 15 minutes or “five minutes following a flatline indication on the EKG, whichever is longer,” according to the state protocol.

Some states are looking for new ways to execute inmates because the drugs used in lethal injections, the most common execution method in the United States, are increasingly difficult to find.

Three states — Alabama, Mississippi and Oklahoma — have authorised nitrogen hypoxia as an execution method, but no state has attempted to use the untested method until now.

Alabama Attorney General Steve Marshall said on Wednesday night that he believes the courts will allow the execution to proceed.

He said: “My office stands ready to carry on the fight for Liz Sennett.

“Two courts have now rejected Smith’s claims. I remain confident that the Supreme Court will come down on the side of justice, and that Smith’s execution will be carried out.”

The victim’s son, Charles Sennett Jr, said in an interview with WAAY-TV that Smith “has to pay for what he’s done”.

“And some of these people out there say, ‘Well, he doesn’t need to suffer like that.’ Well, he didn’t ask Mama how to suffer?” Mr Sennett Jr said. “They just did it. They stabbed her — multiple times.”

The state has predicted the nitrogen gas will cause unconsciousness within seconds and death within minutes.

A state attorney told the 11th Circuit that it will be “the most painless and humane method of execution known to man”.

But some doctors and organisations have raised alarm about the state’s plan.

Robin M Maher, executive director of the Death Penalty Information Centre, said: “It’s indefensible for Alabama officials to simply dismiss the very real risks this untested method presents and experiment on a man who has already survived one execution attempt.”

Much of what is known about death by nitrogen gas comes from industrial accidents or suicide attempts.

Dr Philip Nitschke, a euthanasia expert who designed a suicide pod using nitrogen gas and appeared as an expert witness for Smith, said nitrogen can provide a peaceful, hypoxic death, but said he has concerns about Alabama’s proposal to use a mask.

Dr Nitschke told The Associated Press that Smith’s facial hair, jaw movements and involuntary movements as he feels the effect of the nitrogen could impact the seal.

If there are leaks, Smith could continue to draw in enough oxygen, “to prolong into what could be a very rather macabre, slow process of slowly not getting enough oxygen,” Dr Nitschke said.

He said he could envision scenarios where the execution goes quickly or seriously awry.

Mr Marshall’s office noted in court filings that Smith previously suggested nitrogen as an alternative method when fighting attempts to execute him by lethal injection.

Courts require inmates challenging execution methods to suggest another available alternative. Alabama at the time had not developed a nitrogen protocol.

Robert Grass, an attorney for Smith, told federal courts that they are challenging the specific way the state plans to administer the nitrogen. They argued the use of a gas mask puts Smith at risk of a prolonged and painful death or choking to death on his own vomit.

In 2020 euthanasia guidelines, the American Veterinary Medical Association wrote nitrogen hypoxia is not an acceptable euthanasia method for most mammals because the anoxic environment “is distressing”.

Experts appointed by the United Nations Human Rights Council cautioned they believe the execution method could violate the prohibition on torture.

Mrs Sennett, 45, was found dead on March 18 1988 in her home in Colbert County.

She had eight stab wounds in the chest and one on each side of her neck, according to the coroner.

Her husband, Charles Sennett Sr, killed himself when the investigation focused on him as a suspect, according to court documents.

John Forrest Parker, the other man convicted in the slaying, was executed in 2010.

In 1996, a jury recommended a life sentence by 11-1, but a judge overrode the recommendation and sentenced Smith to death.

Alabama no longer allows a judge to override a jury’s death penalty decision.