MONTGOMERY, Ala. — An Alabama inmate is scheduled to become the second person executed by nitrogen gas in the United States, an execution method that has drawn international scrutiny over human rights concerns.

"Although I have no current plans to grant clemency in this case, I retain my authority under the Constitution of the State of Alabama to grant a reprieve or commutation, if necessary, at any time before the execution is carried out," Alabama Gov. Kay Ivey wrote in her letter to the state Department of Corrections Commissioner John Hamm.

Alan Eugene Miller was set to die by lethal injection in September 2022 but staff could not gain access to his veins for the IV lines before his death warrant expired. Ivey, who has never halted an execution or commuted a death sentence during her two terms as governor, set Miller's execution date for late September.

Alabama was the first to deploy nitrogen hypoxia as an execution method earlier this year over the objections of some human rights groups who warned it could amount to cruel and unlawful punishment. Kenneth Eugene Smith was the first person in the U.S. — and likely the world — to be put to death by nitrogen hypoxia in January after a drawn-out legal battle over Alabama's execution methods.

Convicted for 1999 murders

Miller, now 59, was convicted of killing three people — Lee Holdbrooks, Scott Yancy, and Terry Jarvis — during a pair of 1999 workplace shootings in suburban Birmingham, Alabama. Prosecutors said Miller killed Holdbrooks and Yancy at one business and then drove to another location to shoot Jarvis. Each man was shot multiple times.

Testimony indicated Miller believed the men were spreading rumors about him. Jurors convicted Miller after 20 minutes of deliberation and then recommended a death sentence, which a judge imposed.

Alabama's attorney general’s office decides which condemned inmate is to be executed. The Alabama Supreme Court then authorizes the execution and Ivey schedules the execution date.

Miller has filed a federal lawsuit seeking to bar the use of nitrogen hypoxia as the method of execution with the suit claiming it amounts to cruel and unusual punishment, which the U.S. Constitution bars. The death warrant issued by Ivey will be in effect for 30 hours from midnight Sept. 26 to 6 a.m. Sept 27.

Nitrogen hypoxia:Why Alabama's execution of Kenneth Smith stirs ethical controversy.

Alabama first to deploy nitrogen hypoxia execution method in U.S.

On Jan. 25, Smith became the first person in the nation executed using the method at the William C. Holman Correctional Facility in Atmore, Alabama. In the weeks before the execution, the attorney general’s office wrote in court documents that the inmate would lose consciousness in a matter of “seconds” and die in a matter of minutes.

The execution took about 22 minutes. Smith appeared to convulse and shake vigorously for about four minutes after the nitrogen gas apparently began flowing through his mask. It was another two to three minutes before he appeared to lose consciousness, all while gasping for air to the extent that the gurney shook several times.

The morning after Smith’s execution, Attorney General Steve Marshall described the execution as “textbook” and said the state was ready to carry out more nitrogen hypoxia executions.

'Contractual obligations':This state could be next to use nitrogen gas for death penalty if bill passes

What is nitrogen hypoxia?

Nitrogen hypoxia is a form of execution in which an inmate is deprived of oxygen until they breath only nitrogen, causing asphyxiation. Nitrogen, a colorless, odorless gas, makes up about 80% of the air people breathe. It isn't deadly until it is separated from oxygen.

The United Nations has raised concerns about the execution method, saying it would likely violate the 1984 Convention against Torture, which the U.S. ratified in 1994. Experts previously told USA TODAY the execution method can cause severe pain. The gas can induce seizures or vomiting, which can cause the person to choke to death.

If the mask is not secured well, the nitrogen could leak out and pose a safety hazard to witnesses in the chamber.

"We are concerned that nitrogen hypoxia would result in a painful and humiliating death," U.N. experts wrote in January before Smith's execution.

Contributing: Jeanine Santucci, Thao Nguyen, Cybele Mayes Osterman; USA TODAY