MONTGOMERY, Ala. >> Alabama, unless blocked by the U.S. Supreme Court, will attempt to put an inmate to death with nitrogen gas tonight, 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 couldn’t connect an IV line, is scheduled to be executed at a south Alabama prison.

Alabama plans to put a 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.

“It’s terrifying, This is a situation where we have absolutely no idea what we are walking into,” the Rev. Jeff Hood, Smith’s spiritual adviser and a death penalty opponent, said this morning.

Smith’s attorneys today asked the U.S. Supreme Court to halt the execution to review claims that the new method violates the constitutional ban on cruel and unusual punishment and deserves more legal scrutiny before it is used on a person.

“There is little research regarding death by nitrogen hypoxia. When the State is considering using a novel form of execution that has never been attempted anywhere, the public has an interest in ensuring the State has researched the method adequately and established procedures to minimize the pain and suffering of the condemned person,” Smith’s attorneys wrote.

On Wednesday, appeals court judges rejected Smith’s argument that it would be unconstitutional to make another attempt to execute him after the failed lethal injection.

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 to kill Sennett on behalf of her pastor husband, who was deeply in debt and wanted to collect on insurance.

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

“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,” Marshall said.

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.

The state has predicted the nitrogen gas will cause unconsciousness within seconds and death within minutes. A state attorney told the 11th Circuit Court of Appeals that it will be “the most painless and humane method of execution known to man.”

The execution is scheduled to begin at 6 p.m. local time but could be delayed because of legal action or preparation. The state has until 6 a.m. Friday to carry out the execution.

Some states are looking for new ways to execute inmates because the drugs used in lethal injections have become difficult to find. Three states — Alabama, Mississippi and Oklahoma — have authorized nitrogen hypoxia as an execution method, but no state has attempted to use the untested method until now.

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?” the son said. “They just did it. They stabbed her — multiple times.”

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

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.

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

Smith’s attorneys have raised concerns that Smith could choke to death on his own vomit as the nitrogen gas flows. The state made a last-minute change to procedures to say he will not be allowed food in the eight hours leading up to the execution.

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

Smith’s 1989 conviction was overturned. He was convicted again in 1996. The 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.