The U.S. Supreme Court has declined to intervene in Thursday’s execution of Alabama convict Kenneth Smith, who’s slated to become the first person in the country put to death with nitrogen gas.

No opinion was issued by justices who decided against reviewing Smith’s case the day before his scheduled execution.

The U.S. 11th Circuit Court flat-out rejected Smith’s argument against execution by nitrogen hypoxia Wednesday. The Supreme Court could have still decided to review that federal appellate court’s ruling, according to Alabama outlet AL.com.

Mississippi and Oklahoma joined Alabama in legalizing execution by nitrogen gas, but no state has so far put an inmate to death that way.

Smith’s legal team has contended the 58-year-old prisoner is being treated as a “test subject,” which violates the Constitution’s barring of cruel and unusual punishment. Nitrogen hypoxia critics contend that putting a mask on a person’s face that forces them to breath nitrogen is tantamount to suffocation.

Alabama’s attorney general argued that that form of execution will render Smith unconscious almost immediately, meaning he would effectively die in his sleep.

This isn’t the first time Alabama has tried to execute Smith, who was convicted in connection with a 1988 murder-for-hire plot that left the wife of a rural pastor. He was set to be put to death by lethal injection in November 2022, but state officials were unable to tap into a vein to spread poison through his system.