A man found guilty of murdering a woman nearly 37 years ago was executed on Tuesday evening in Alabama, marking the country's sixth execution using nitrogen gas.

Gregory Hunt was declared dead at 6:26 p.m. on Tuesday at a prison in southern Alabama. Strapped to a gurney with a blue-rimmed mask covering his face, Hunt didn't utter any final words but seemed to give a thumbs-up and a peace sign with his fingers before the gas was released.

The exact moment when the gas started flowing remains unclear.

Hunt briefly shook, gasped for air and lifted his head off the gurney. He let out a moan around 5:59 p.m., reports . and raised his feet.

READ MORE:

READ MORE:

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.

The execution in Alabama is one of four that had been scheduled this week across the United States. Executions are also due in Florida and South Carolina.

A judge in Oklahoma issued a temporary stay for an execution in that state on Monday, but the state attorney general is seeking to have it lifted.

Hunt was convicted of killing Karen Lane, a woman he had been seeing for about a month, according to court records. Lane was 32 when she was murdered on Aug. 2, 1988, in the Cordova flat she shared with a woman who was Hunt's cousin.

Prosecutors claimed that Hunt broke into her flat, sexually abused her and then murdered her. The conducted the post-mortem examination testified that she died from blunt force trauma and had sustained around 60 injuries, including 20 to the head.

On June 19, 1990, a jury found Hunt guilty of capital murder during sexual abuse and burglary. The jurors recommended by an 11-1 vote that he should receive a death sentence, which was subsequently imposed by a judge.

Hunt's final plea for a stay of execution, which he filed himself, centred on allegations that prosecutors made false statements to jurors about evidence of sexual abuse. The element of sexual abuse is what escalated the crime to a death penalty offence.

In a submission to the U.S. Supreme Court, Hunt, acting as his own lawyer, wrote that a prosecutor told jurors that cervical mucus was found on a broomstick near Hunt's body. However, the victim did not have a cervix due to a previous hysterectomy.

The Alabama attorney general's office dismissed the claim as baseless and said even if the prosecutor made a mistake in that statement, it did not cast doubt on the conviction.

Speaking from prison last month via telephone, Hunt did not deny killing Lane but insisted he did not sexually assault her. He also portrayed himself as someone who had been transformed by prison.

"Karen didn't deserve what happened to her," Hunt stated.

Hunt admitted he had been drinking and taking drugs on the night of the crime and became jealous when he saw Lane in a car with another man.

"You have your come-to-Jesus moment. Of course, after the fact, you can't believe what has happened. You can't believe you were part of it and did it," Hunt remarked.

Born in 1960 and having found himself on death row since 1990, Hunt now ranks as one of the longest-serving inmates in Alabama's correction system. He described prison as his "hospital" for mending a fractured psyche and mentioned that, since 1988, he has conducted a Bible study group that draws upwards of two dozen fellow prisoners.

"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 shared about his experience.

"The way she was killed is just devastating," recounted Denise Gurganus, Lane's sister, to WBRC television during a 2014 vigil honouring victims of crime. "It's hard enough to lose a family member to death, but when it's this gruesome."

In their plea for justices to deny Hunt's request to postpone his execution, the Alabama attorney general's office pointed out the harsh truth that Hunt has lived on death row longer than Lane had been alive.

In a controversial milestone, Alabama stood as the first US state to perform an execution by nitrogen gas last year. To date, nitrogen has been employed in five executions – four within Alabama's borders and one in Louisiana.

The method revolves around issuing a gas mask to force an individual to inhale pure nitrogen, effectively cutting off the oxygen necessary for survival.

Hunt had identified nitrogen as his chosen method of execution. He made this choice before Alabama had established procedures for using gas.

Alabama also permits inmates to opt for lethal injection or the electric chair.