ATMORE, Ala. (AP) — An Alabama man convicted of killing a woman in 1988 was put to death Tuesday evening in the nation’s sixth execution by nitrogen gas.

Gregory Hunt was pronounced dead at 6:26 p.m. at a south Alabama prison, authorities said, one of four scheduled this week in the United States.

Strapped to a gurney with a blue-rimmed mask covering his entire face, Hunt gave no final words but appeared to give a thumbs-up sign and a peace sign with his fingers before the gas started flowing. It was not clear when the gas began.

Hunt briefly shook, gasped and raised his head off the gurney. He let out a moan at about 5:59 p.m. and raised his feet. He took a series of four or more gasping breaths with long pauses in between, and made no visible movements after 6:05 p.m.

Hunt was convicted of capital murder for the killing of Karen Lane, who was 32 when she was killed on Aug. 2, 1988, in the Cordova apartment she shared with another woman in Walker County.

Hunt had dated Lane for about a month. Prosecutors said that after becoming enraged with jealousy, he broke into Lane’s apartment and sexually abused her and beat her to death, inflicting 60 injuries on her body. Jurors convicted him in 1990 and recommended a death sentence by an 11-1 vote.

In a statement, Gov. Kay Ivey said Lane experienced an “unimaginable final hours of her young life.”

“Tonight, the state carried out the lawfully imposed punishment for Gregory Hunt who is undeniably guilty,” Ivey said.

Hunt, who was born in 1960, was among the longest-serving inmates on Alabama’s death row. He told The Associated Press last month that finding religion in prison helped him get “free of my poisons and demons” and that he tried to help other inmates.

“Just trying to be a light in a dark place, trying to tell people if I can change, they can too … become people of love instead of hate,” he said.

Lane’s sister declined to comment when reached by telephone this week.

But in 2014 at a vigil for crime victims, she said, “The way she was killed is just devastating.”

“It’s hard enough to lose a family member to death, but when it’s this gruesome,” she said.

Hunt acted as his own attorney in a filing to the U.S. Supreme Court seeking to halt the execution, arguing that prosecutors misled jurors about the evidence of sexual abuse.

The Alabama attorney general’s office called the claim meritless.

Last year Alabama became the first state to carry out an execution with nitrogen gas. The method has now been used in six executions — five in Alabama and one in Louisiana.

Hunt selected nitrogen gas over the other options, lethal injection or the electric chair, before Alabama developed procedures for the method.