MONTGOMERY, Ala. (AP) — An Alabama inmate set to be the nation’s first person ever put to death by nitrogen gas will ask a federal appeals court Friday to block the upcoming execution using the untested method.

Kenneth Smith, 58, is scheduled to be executed Thursday, when a respirator-type mask will be placed on his face to replace his breathing air with pure nitrogen — depriving him of the oxygen needed to stay alive. Three states — Alabama, Oklahoma and Mississippi — have authorized nitrogen hypoxia as an execution method, but no state has previously attempted to use it.

The 11th U.S. Circuit Court of Appeals will hear arguments Friday afternoon, when Smith’s lawyers will appeal a federal judge’s Jan. 10 decision to let the execution go forward, arguing that Alabama is trying to make Smith the “test subject” for an experimental execution method after he survived the state’s previous attempt to put him to death by lethal injection in 2022. They contend that the new nitrogen hypoxia protocol is riddled with unknowns and potential problems that could subject him to an agonizing death.

“Because Mr. Smith will be the first condemned person subject to this procedure, his planned execution is an experiment that would not be performed or permitted outside this context,” Smith’s attorneys wrote in the Monday court filing. They also argued that the state violated his due process rights by scheduling the execution when he has pending appeals.

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Alabama Attorney General Steve Marshall’s office will ask the court to let the execution proceed. The state called Smith’s concerns speculative and has predicted the nitrogen gas will “cause unconsciousness within seconds, and cause death within minutes.”

“Smith admits that breathing 100% nitrogen gas would result in … death. And the experts agree that nitrogen hypoxia is painless because it causes unconsciousness in seconds,” the state argued.

Lethal injection is the most commonly used execution method in the United States, but as the drugs have become more difficult to obtain, states have looked to alternate methods. If Smith’s execution by nitrogen hypoxia is carried out, it will be the first new execution method used in the United States since lethal injection was first used in 1982.

U.S. District Judge R. Austin Huffaker rejected Smith’s bid for an injunction to stop the execution earlier this month. Huffaker acknowledged that execution by nitrogen hypoxia is a new method but noted that lethal injection — now the most common execution method in the country — was also new once.

Smith was one of two men convicted of the 1988 murder-for-hire of a preacher’s wife. Prosecutors said Smith and the other man were each paid $1,000 to kill Elizabeth Sennett on behalf of her husband, who was deeply in debt and wanted to collect insurance. John Forrest Parker, the other man convicted in the case, was executed by lethal injection in 2010. Sennett’s husband killed himself when the murder investigation focused on him as a suspect, according to court documents.

Alabama attempted to execute Smith by lethal injection in 2022 but the state called off the execution before the lethal drugs were administered because authorities were unable to connect the two required intravenous lines to Smith’s veins. Smith was strapped to the gurney for nearly four hours during that execution attempt, his lawyers said.

In a separate case, Smith has also argued that after surviving one execution attempt it would violate the federal ban on cruel and unusual punishment for the state to make a second attempt to execute him. Smith on Friday asked the U.S. Supreme Court to stay the execution to consider that question. The filing came after the Alabama Supreme Court rejected Smith’s claim in a ruling last week.