A death row inmate convicted of killing three coworkers over rumors about his sexuality is set to be executed with nitrogen gas in Alabama on Thursday, making him just the second inmate in the United States to face the controversial method described by one witness as "horrific."

Alan Eugene Miller, 59, was convicted of murdering Terry Jarvis, Lee Holdbrooks and Christopher Yancy in two workplace shootings on the morning of Aug. 5, 1999, in the suburban Birmingham city of Pelham.

If his execution moves forward as expected, he will become the fourth inmate executed in Alabama in 2024 and the 18th in the nation. He's also among five men being executed in the U.S. in just a six-day period: South Carolina executed Freddie Owens on Friday, and on Tuesday Missouri executed Marcellus Williams less than an hour before Texas executed Travis James Mullis.

Miller was supposed to be executed by lethal injection in September 2022 but staff could not find his veins for an IV line before the death warrant expired, therefore the inmate was poked with needles for over an hour while vertically strapped to a gurney.

Here's what you need to know about Miller's case, who his victims were and other details of his execution.

Who is Alan Eugene Miller?

Miller was born on Jan. 20, 1965, and grew up with a long history of mental illness in his family, tracing back at least four generations, according to a federal appeals document filed in January 2013. 

At a young age, Miller's family constantly moved and he experienced "severe poverty," the court record says. When his parents divorced, his mother and siblings spent years on welfare and food stamps, the document continued.

Miller's mother, Babara, remarried his father for financial reasons but court records say he struggled to keep a stable job and frequently thought people were plotting against him, including his coworkers.

Barbara Miller described the homes that her family lived in as “junky, rat-infested, roach-infested, just falling in," records say.

Before the murder conviction, Miller had a "nonviolent nature" and a "good employment history," according to his attorney.

What was Alan Eugene Miller convicted of?

An Alabama jury found Miller guilty of shooting Holdbrooks six times, Jarvis five times and Yancy three times. He initially shot and killed Holdbrooks and Yancy, his coworkers at Ferguson Enterprises, before driving to Post Airgas, a welding supply store, to murder Jarvis.

Pelham police arrested Miller during a traffic stop on the same day. Once Miller was handcuffed, officers say they found a Glock lying on the driver's seat and an empty ammunition magazine on the passenger seat, according to court documents.

Despite Miller being deemed "mentally ill" by a forensic psychiatrist during his sentencing hearing, the court ordered him to be executed for his crimes, finding that his condition didn't meet the level of mania necessary to establish an insanity defense under Alabama law.

"Miller suffered from a delusional disorder that substantially impaired his rational ability," according to the court document. "This delusional disorder − coupled with Miller's history as a loner − resulted in Miller's believing the people he worked with talked about him and that they had spread rumors about him. Miller believed that Terry Jarvis had told other employees at Post Airgas that Miller was a homosexual."

Who were Terry Jarvis, Lee Holdbrooks and Christopher Yancy?

Sherry Lawley told USA TODAY that her brother, Terry Jarvis, was the kindest and most generous person she knew.

"He would give you the shirt off of his back if he could," said Lawley, a resident of Maylene in central Alabama. "He would do anything for anybody … Terry was my baby brother, but at some point in life, it seemed like I became his baby sister because he took care of me."

After he was killed, she said: "It was just like a piece of my heart was missing that I won't ever get back."

As for Yancy, he was a faith-driven father of two and a "big teddy bear type," according to the Rev. Chris Joiner, the pastor of the Alabama church that Yancy and his family attended.

"It was a strong family," Joiner told USA TODAY. "They were very loving and attentive to one another."

Yancy's family suffered another tragedy following his murder when his wife, Kim, was killed at the age of 38 in a car accident in 2012, according to the Shelby County Reporter.

USA TODAY could not reach Holdbrooks' family, but according to his obituary, he died a married man without children.

When and where will Alan Eugene Miller be executed?

Miller will be executed at about 6 p.m. CT on Thursday in the death chamber at the William C. Holman Correctional Facility in Atmore, a small town about 130 miles south of Montgomery.

More about the execution method

Miller is scheduled to become the second death row inmate to be executed using nitrogen gas in the U.S.

The first was in January with the execution of 58-year-old Kenneth Smith, who "appeared to convulse and shake vigorously for about four minutes" after the nitrogen gas began flowing, according to reporter Marty Roney, who witnessed the execution for the Montgomery Advertiser, part of the USA TODAY Network.

Roney will also be a witness at Miller's execution and will report on his observations. Check back with USA TODAY about what he sees.

The Rev. Jeff Hood, a spiritual advisor who has witnessed more than a half dozen executions, was at Smith's and described it as being by far "the most violent" he's seen.

"We're talking about minutes and minutes of thrashing and spitting," Hood told USA TODAY. "His head going up and down (and) back and forth. The (expletive) gurney that's bolted to the floor started shaking."

Miller weighs around 400 pounds and is what Hood called a "shell of a human being."

"You've a got a situation where you're going to strap a 400-pound man to a gurney, suffocate him to death and expect that to go well," the reverend said, adding that this type of execution brings society closer to a "moral apocalypse."

What will Alan Eugene Miller's last meal be?

The Alabama Department of Corrections will release Miller's last meal on execution day.

Who will witness Alan Eugene Miller's execution?

The Alabama Department of Corrections will release the general witness list on the night of the execution, but Lawley said she will not be attending.

"I've always said 'I wanted to be there,'" Lawley said. "Over the years, I've changed my mind about that. I feel sorry for his (Miller's) parents."

Members of the media will be at the execution, including:

• The Montgomery Advertiser, part of the USA TODAY Network
• CBS42
• The Associated Press
• AL.com
• WBRC/Fox 6 News

When is the nation's next execution?

The next execution in the U.S. is scheduled for Oct. 1 in Texas. Garcia Glen White, a former college football player at Lubbock Christian University, is set to be executed by lethal injection following his conviction for the 1989 murder of identical twin 16-year-old girls in Houston.

Contributing: Marty Roney/Montgomery Advertiser