MONTGOMERY, Ala. (AP) — Alabama is preparing to carry out the nation’s second nitrogen gas execution on Thursday as disagreements continue over the humaneness of the new method of putting prisoners to death.

Alan Eugene Miller, 59, is scheduled to be executed with nitrogen gas at a south Alabama prison. Miller was convicted of killing three men — Lee Holdbrooks, Christopher Scott Yancy and Terry Jarvis — in back-to-back workplace shootings in 1999.

Alabama in January put Kenneth Smith to death in the first nitrogen gas execution. The new execution method involves placing a respirator gas mask over the inmate’s face to replace breathable air with pure nitrogen gas, causing death by lack of oxygen.

Alabama officials and advocates have argued over whether Smith suffered an unconstitutional level of pain during his execution. He shook in seizure-like spasms for more than two minutes as he was strapped to the gurney. That was followed by several minutes of gasping breathing.

“Alabama’s nitrogen hypoxia system is reliable and humane,” Alabama Attorney General Steve Marshall said last month in announcing a lawsuit settlement agreement that allowed for Miller’s execution. The state has scheduled a third nitrogen execution for November.

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But death penalty opponents and advocates for other inmates facing nitrogen execution maintain that what happened with Smith shows there are problems with, or at least questions about, the new execution method. They said the method should be scrutinized more before it is used again.

“The fact that the state scheduled two more nitrogen executions without publicly acknowledging the failures of the first one is concerning. Going through with a second in the world nitrogen execution without reassessing the first, and under a continued veil of secrecy is not how a transparent government operates,” John Palombi, an attorney with the Federal Defenders Program who is representing another inmate facing a nitrogen execution in November, wrote in an email.

Death penalty opponents on Wednesday delivered petitions asking Gov. Kay Ivey to halt the execution. Miller is one of five death row inmates scheduled to be put to death in the span of one week, an unusually high number of executions that defies a yearslong trend of decline in the use of the death penalty in the U.S.

Miller, a delivery truck driver, was convicted of capital murder for the shootings that claimed three lives and shocked the city of Pelham, a suburban city just south of Birmingham.

The Aug. 5, 1999, workday had begun normally, a witness testified, until Miller showed up armed with a handgun saying he was “tired of people starting rumors on me.”

Police say that early that morning Miller entered Ferguson Enterprises and shot and killed two coworkers: Holdbrooks, 32, and Yancy, 28. He then drove 5 miles (8 kilometers) away to Post Airgas, where he had previously worked, and shot Jarvis, 39.

All three men were shot multiple times. A prosecutor told jurors at the 2000 trial that the men “are not just murdered, they are executed.”

Miller had initially pleaded not guilty by reason of insanity but later withdrew the plea. A psychiatrist hired by the defense said that Miller was mentally ill, but he also said Miller’s condition wasn’t severe enough to use as a basis for an insanity defense, according to court documents.

Jurors convicted Miller after 20 minutes of deliberation and voted he receive the death penalty.

Alabama had previously attempted to execute Miller by lethal injection. But the state called off the execution after being unable to connect an IV line to the 351-pound (159-kilogram) inmate. The state and Miller agreed that any other execution attempt would be with nitrogen gas.

The state might be making minor adjustments to execution procedures. Miller had initially challenged the nitrogen gas execution plans, citing witness descriptions of what happened to Smith. But he dropped the lawsuit after reaching a settlement last month with the state.

Court records did not disclose the terms of the agreement, but Miller had suggested several changes to the state’s nitrogen gas protocol. Those included using medical grade nitrogen and a sedative beforehand. Will Califf, a spokesperson for Attorney General Marshall, last month said he could not confirm if the state had agreed to make changes to execution procedures.

Mara E. Klebaner, an attorney representing Miller, said last month that he “entered into a settlement on favorable terms to protect his constitutional right to be free from cruel and unusual punishments.”