ATMORE, Ala. (AP) — Alabama used nitrogen gas Thursday to execute a man convicted of killing three people in back-to-back workplace shootings, the second time the method that has generated debate about its humaneness has been used in the country

Alan Eugene Miller, 59, was pronounced dead at 6:38 p.m. local time at a south Alabama prison. He shook and trembled on the gurney for about two minutes with his body at times pulling against the restraints. That was followed by about six minutes of periodic gulping breaths before he became still.

Miller was convicted of killing three men — Lee Holdbrooks, Christopher Scott Yancy and Terry Jarvis — in 1999 and the state had previously attempted to execute him by lethal injection in 2022.

“I didn't do anything to be in here,” Miller said in his final words that were at times muffled by the blue-rimmed gas mask that covered his face from forehead to chin. However, witnesses at the trial had expressed no doubt about his guilt, describing Miller shooting the three men.

At the execution, Miller also asked his family and friends to “take care” of someone but it was not clear whose name he said.

Miller was one of five inmates put to death in the span of one week, an unusually high number that defies a yearslong trend of decline in the use of the death penalty in the U.S.

“Tonight, justice was finally served for these three victims through the execution method elected by the inmate," Alabama Gov. Kay Ivey said in a statement. “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.”

Family members of the three victims did not witness the execution and did not issue a statement to be read to reporters, state officials said.

The execution was the second to use the new method Alabama first employed in January, when Kenneth Smith was put to death. The 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 after he shook in seizure-like spasms for several minutes, at times rocking the gurney. Smith then gasped for breath for several minutes. The shaking exhibited by Miller was similar to what was seen at the first nitrogen gas execution but did not seem as long or as violent.

Alabama Corrections Commissioner John Q. Hamm said the shaking movements were anticipated.

“Just like in Smith we talked about there is going to be involuntarily body movements as the body is depleted of oxygen. So that was nothing we did not expect,” Hamm said, explaining the nitrogen gas flowed for 15 minutes. “Everything went according to plan and according to our protocol."

Alabama Attorney General Steve Marshall said the execution “went as expected and without incident.”

“Tonight, despite misinformation campaigns by political activists, out-of-state lawyers, and biased media, the State proved once again that nitrogen hypoxia is both humane and effective,” Marshall said in a statement.

Marshall did not personally witness the execution but a representative from his office did, prison officials said.

A delivery truck driver, Miller was convicted of capital murder for the Aug. 5, 1999, shootings that claimed three lives and shocked the city of Pelham, a suburban city just south of Birmingham.

Police say that early that morning, Miller entered Ferguson Enterprises and fatally shot two co-workers: 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. Trial testimony indicated that Miller was paranoid and believed his co-workers had been gossiping about him.

“You’ve been spreading rumors about me,” a witness described Miller as saying before he opened fire. All three men were shot multiple times.

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 his 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 recommended by a vote of 10-2 that he receive the death penalty.

In 2022, the state called off the previous attempt to execute Miller after being unable to connect an IV line to the 351-pound (159-kilogram) inmate. Miller had initially challenged the nitrogen gas protocol but dropped his lawsuit after reaching an undisclosed settlement with the state.

Hamm said the state did not change the protocol. Miller, among other things, had requested to be given a sedative. Hamm declined to say if Miller was given a sedative and referred questions about the settlement to Miller’s attorneys.