(CNN) — Alabama is poised to carry out the first known execution using nitrogen gas Thursday to put Kenneth Smith to death for his role in a 1988 murder for hire.

Smith, who survived the state’s initial attempt to execute him by lethal injection in 2022, is tentatively set to die Thursday evening as the state endeavours to use a wholly new method of execution some experts are concerned could cause him to experience excessive pain or torturous death.

Little is known about how the method, known as nitrogen hypoxia, will be carried out because the state’s published protocol bears redactions experts say shield key details from public scrutiny.

The state, in court records, indicated the redactions were made to maintain security.

The US Supreme Court on Wednesday declined to intervene in Smith’s case after his attorneys tried to argue a second execution attempt would violate the US Constitution’s protection against cruel and unusual punishment.

A separate federal appeals court ruling on Wednesday evening also declined to halt the execution.

Smith’s team again Thursday morning appealed to the US Supreme Court and requested a stay of execution.

State officials on Wednesday welcomed the high court’s rejection of Smith’s earlier request, framing it as an “attempt to bar the State from executing him by any method at all,” Attorney General Steve Marshall said in a statement.

Alabama “remains confident that the execution, and long-awaited justice, will proceed as planned,” he said.

Still, critics of the state and Smith’s advocates fear the nitrogen gas execution could go awry, pointing to its novel nature, questions around its shrouded protocol and Alabama’s recent struggles to carry out lethal injections.

In 2022 alone, the state carried out or attempted three executions in a row that critics deemed “botches,” meaning they deviated from the stated protocol. In two of the cases, the inmates – including Smith – survived as officials called off the executions because they could not set an intravenous line used to deliver the fatal drugs before the death warrants expired.

“It’s lunacy, absolute lunacy,” said Smith’s spiritual adviser, the Rev. Jeff Hood, who will be in the execution chamber when the inmate is put to death.

“The process, obviously, is designed to execute Kenneth Smith,” Hood told CNN.

“But the way that they’re constructing this, the way that they’re doing it, the way that they’re being silent, the way that they’re holding back information, yes, it’s incredibly concerning. And should be incredibly concerning for everybody in the room.”

Smith’s jury voted 11-1 for life sentence

Smith was convicted and sentenced to death for the murder of Elizabeth Sennett, whose husband, Charles, was having an affair and had taken out an insurance policy on his wife, according to court records.

Charles Sennett recruited a man who recruited two others, including Smith, and agreed to pay each $1,000 to kill his wife and make it look like she died in a burglary, the records show. The men carried out the killing as planned in March 1988, and Smith took from the Sennett home a VCR player that he stored in his own home.

Charles Sennett killed himself a week after the murder, records state, as the investigation’s focus turned to him. Smith was ultimately arrested after authorities, based on an anonymous tip, searched his home and found the VCR player.

Smith was convicted and sentenced to die, but an appeals court overturned the initial outcome and ordered a new trial. He was again convicted in the retrial, but this time his jury voted 11-1 for a sentence of life in prison without the possibility of parole.

The judge in Smith’s second trial, however, essentially vetoed the jury’s vote and sentenced the defendant to death – a practice known as judicial override that’s since been repealed in Alabama.