Alan Eugene Miller was executed Thursday evening in Alabama, state officials said, making him the second inmate known to die by nitrogen hypoxia, a controversial method critics say is tantamount to torture.

Miller, 59, who was sentenced to death in 2000 for the 1999 killings of three men, was pronounced dead at 6:38 p.m. at a prison in Atmore, Alabama Department of Corrections Commissioner John Hamm said at a news conference.

Miller shook and trembled on a gurney for about two minutes, with his body at times pulling against restraints, according to The Associated Press, which had a reporter witness the procedure. The shaking and trembling was followed by about six minutes of periodic gulping breaths before he became still, the AP reported.

“I didn’t do anything to be in here,” Miller said in his final words, which at times were muffled by a mask that covered his face from forehead to chin, according to the AP.

Miller was fitted with the mask during the procedure, during which nitrogen gas flowed for about 15 minutes, Hamm said. In response to a reporter’s question, Hamm confirmed the two minutes of shaking, which he said was to be expected.

“There’s going to be involuntary body movements as the body is depleted of oxygen. That is nothing we did not expect,” Hamm said at the news conference.

“Everything went according to plan and according to our protocol, so it went just as we had planned,” Hamm said.

At one point, a corrections officer had to adjust Miller’s mask, Hamm confirmed in response to a reporter’s question. “That’s just making sure the mask is fitted,” Hamm said.

“Tonight, justice was finally served for these three victims through the execution method elected by the inmate,” Alabama Gov. Kay Ivey said in a release. “His acts were not that of insanity, but pure evil. Three families were forever changed by his heinous crimes, and I pray that they can find comfort all these years later.”

Miller’s execution came after a yearslong chain of events surrounding how he would be put to death: He first requested death by nitrogen hypoxia, but the state said it wasn’t prepared to use the method, and it then attempted to execute him by lethal injection in September 2022. That attempt, however, was called off, with state officials saying they couldn’t access Miller’s veins before the execution warrant expired.

The state subsequently agreed not to execute Miller using any method other than nitrogen hypoxia. But then Alabama executed Kenneth Smith early this year by nitrogen hypoxia in what is believed to be the first execution by that method. Witnesses said Smith was shaking and writhing on the gurney for minutes before dying.

Miller then challenged the state’s nitrogen hypoxia protocol in a federal lawsuit, claiming it could cause him undue suffering, thus violating his Eighth Amendment protections against cruel and unusual punishment. The suit, however, was settled last month.

The terms of the settlement were confidential, though state Attorney General Steve Marshall touted it as proof Alabama’s nitrogen gas execution method is constitutional.

“The resolution of this case confirms that Alabama’s nitrogen hypoxia system is reliable and humane,” Marshall said in August.

Proponents of the nitrogen hypoxia execution method, which replaces oxygen breathed by an inmate with 100% nitrogen, say a person would likely lose consciousness shortly into the procedure, making it more humane than other execution methods. However, doctors have said that they could not pinpoint if or when a person will lose consciousness when exposed to high concentrations of nitrogen gas.

CNN has repeatedly reached out to lawyers for Miller for comment on his settled lawsuit and execution.

On Thursday in the hours before the execution, Miller had nine visitors and had a final meal of hamburger steak, baked potato and French fries, the Alabama Department of Corrections said.

The 1999 crime

Miller has been staring down the end of his life for more than two decades. He was sentenced to death in 2000 for the 1999 murders of Lee Holdbrooks, Scott Yancy, and Terry Lee Jarvis.

Miller had worked with each of the victims and became upset when he believed the three “spread rumors about him,” according to a release from the Alabama attorney general’s office.

On the morning of August 5, 1999, Miller shot two of the three men at Ferguson Enterprises in Pelham, Alabama, according to court documents.

“I’m tired of people starting rumors on me,” Miller said, armed with a pistol while walking out of his employer’s office, court documents say.

Yancy was shot three times, according to court documents, and was unable to move after the first shot, “traveled through his groin and into his spine, paralyzing him.”

Holdbrooks was shot six times and tried to crawl down a hallway to escape before Miller shot him in the head, “causing him to die in a pool of blood,” the documents say.

After killing Holdbrooks and Yancy, Miller headed to his previous employer, Post Airgas, where Jarvis worked.

Miller walked in and said, “Hey, I hear you’ve been spreading rumors about me.”

Jarvis replied that he had not been spreading rumors about Miller but moments later, Miller shot Jarvis “a number of times.”

Miller was later captured on the highway, court documents say, with “a Glock pistol with one round in the chamber and 11 rounds in the ammunition magazine.”

A forensic psychiatrist who testified for Miller’s defense determined he was mentally ill and suffering a delusional disorder, leading him to believe the victims were spreading rumors about him. The psychiatrist concluded, however, that Miller’s mental illness didn’t meet the standards for an insanity defense in Alabama.

“I feel that it has taken way too long to get here,” Tara Barnes, Holdbrooks’ widow told CNN Tuesday.

CNN has attempted to reach family members of Yancy and Jarvis.

What is nitrogen hypoxia?

In September 2022, Alabama officials tried to execute Miller by lethal injection, but failed because they could not access his veins within the required time limit.

Miller was scheduled to be executed by lethal injection after a US Supreme Court ruling vacated a lower court injunction in a long-running dispute over whether he would die by that method or nitrogen hypoxia.

Prior to that initial attempt, Miller and his attorneys had fought to ensure he would be executed by nitrogen gas, a method he had previously chosen but the state was not ready to use.

After the failed attempt, Miller was sent back to death row.

Miller and his attorneys filed their lawsuit challenging the state’s nitrogen hypoxia protocol after it was used for the first time in Smith’s execution.

Smith was sentenced to death for his role in a 1988 murder for hire and, like Miller, had previously survived a failed attempt to execute him by lethal injection in 2022.

The process of death by nitrogen gas involves forcing an inmate to inhale 100% nitrogen gas, depriving them of the oxygen needed to survive. But death by nitrogen gas has been criticized as experts have said it could lead to excessive pain or even torture.

During Smith’s execution earlier this year, he appeared conscious for “several minutes,” and for two minutes after that, he “shook and writhed on a gurney,” according to the media witness report.

That was followed by several minutes of deep breathing before his breath began slowing “until it was no longer perceptible for media witnesses,” the media witness report said.

“Clearly it was not the instant, painless death that they promised,” Dr. Jonathan Groner, a professor of surgery at The Ohio State University College of Medicine told CNN last week. “There’s a lot of suggestion that it wasn’t good, wasn’t pleasant.”

United Nations experts “unequivocally condemned” Smith’s execution and the use of nitrogen gas inhalation, saying in a statement it “was nothing short of State-sanctioned torture.”

“The use, for the first time in humans and on an experimental basis, of a method of execution that has been shown to cause suffering in animals is simply outrageous,” the UN experts said.

“The theory is that if you just get rid of all the oxygen, just breathing pure nitrogen, you won’t feel that intense pressure, like you’re holding your breath, right? It doesn’t really work out that way,” Groner, who’s studied capital punishment for more than two decades, said.

While Alabama is the only state that’s put this execution method to the test, it’s not the only state that’s adopted the use of the procedure. Louisiana, Oklahoma and Mississippi have also authorized death by nitrogen hypoxia, according to the Death Penalty Information Center.

CNN’s Dakin Andone and Lauren Mascarenhas contributed to this report