The U.S. Supreme Court has denied a request to halt Alabama’s planned execution of Kenneth Smith Thursday night.

Smith is set to be the first inmate in America to be executed using nitrogen gas. The execution is slated for 6 p.m. CST at William C. Holman Correctional Facility in Atmore.

The justices denied Smith’s lawyer’s request to review the case and for a stay of execution on Wednesday afternoon. No opinion was issued.

Smith’s lawyers had sought the court to review the case while another appeal centering on the method– nitrogen hypoxia– is pending before the U.S. 11th Circuit Court of Appeals. That court has yet to rule on Smith’s arguments.

If the federal appeals court also rules that the untested method of execution can go forward, Smith’s lawyers can again appeal to the nation’s highest court in Washington D.C.

Smith was convicted for his role the murder-for-hire of Elizabeth Dorlene Sennett in her home in Colbert County in 1988. Sennett was a pastor’s wife who was beaten and stabbed. Smith confessed to his role in the crime after the slaying.

He’s set to be put to death by breathing in pure nitrogen through a fitted gas mask. In theory, he will lose consciousness without any source of oxygen and die.

Smith was first set to be executed in November 2022, when the state attempted to execute Smith via lethal injection. But the execution was called off just before midnight, when prison workers couldn’t get set up the intravenous lines needed for the three-drug lethal injection cocktail.

Protestors and anti-death penalty advocates have sought to get the execution called off, asking Gov. Kay Ivey for a stay and rallying in Montgomery earlier this week.