A convicted killer is set to become the first inmate in the US to be executed with nitrogen gas.
Kenneth Eugene Smith, 58, is scheduled to be executed at a south Alabama prison on Thursday night.
The execution method has never been used before and the state of Alabama claims it will be humane, but critics have called it cruel and experimental.
An industrial-type respirator mask is expected to be put over Smith’s face and replace breathing air with pure nitrogen gas, causing him to die from lack of oxygen.
The death 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.
Smith was nearly executed by lethal injection in 2022 because state officials couldn’t find a suitable vein to inject the lethal drugs.
Attorneys for Smith have waged a legal battle to halt the execution, arguing that the state is seeking to make Smith the "test case" for the new execution method that merits more legal scrutiny before it is used on an inmate.
"It's an experiment," said Reverend Jeff Hood, Smith's spiritual advisor and a death penalty opponent.
The US Supreme Court on Wednesday rejected Smith's argument that it would be unconstitutional to make another attempt to execute him after the failed lethal injection.
The U.S. 11th Circuit Court of Appeals on Wednesday night also declined to halt the execution, saying Smith had not sufficiently supported claims the new execution method would violate the constitutional ban on cruel and unusual punishment.
His attorneys are expected to appeal that decision to the U.S. Supreme Court.
Smith is one of two men convicted in the 1988 murder-for-hire killing 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.
Three states — Alabama, Mississippi and Oklahoma — have authorised nitrogen hypoxia as an execution method, but no state has attempted to use the untested method until now.
Alabama Attorney General Steve Marshall said on 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 added.
