The U.S. Supreme Court refused Wednesday to stop the state of Alabama from proceeding with plan to execute an inmate with an untested method – nitrogen hypoxia.

Kenneth Eugene Smith is scheduled to be the first person in the United States to be executed by nitrogen hypoxia. His execution is scheduled for 6 p.m. Thursday at the Holman Correctional Facility in Atmore, Alabama.

Smith’s attorneys had asked the nation’s highest court to stay the execution. The court declined.

Smith’s attorneys also petitioned the 11th U.S. Circuit Court of Appeals to block the execution based on the grounds that the use of nitrogen hypoxia has not been tested. The appeals court has not yet ruled.

Smith was sentenced to death for his role in the 1988 murder-for-hire killing of Elizabeth Dorlene Sinnett.

Alabama tried to execute Smith in 2022 by lethal injection but failed after executioners failed to be able to find a vein quickly enough.

Alabama has approved the use of nitrogen hypoxia — a practice in which lethal doses of pure nitrogen are given to the person until they asphyxiate – after the drugs used for lethal injection have become more difficult to acquire.

National and international groups have protested Alabama’s plans as experimental and potentially painful.