Atmore, Alabama - An Alabama man convicted of the murder of a hitchhiker was put to death by nitrogen gas on Thursday, officials said, the third use of the controversial execution method in the state this year.

The execution of Carey Grayson (49) was conducted at the William Holman Correctional Facility in the town of Atmore, where he was pronounced dead at 6:33 PM local time, the state's prison authority said in a statement.

"Alabama has successfully used nitrogen hypoxia to carry out Carey Grayson's execution," Alabama Attorney General Steve Marshall said in a statement. "Tonight, justice has been served."

Grayson was sentenced to death for the 1994 murder of Vickie Deblieux (37), who was killed while hitchhiking from Tennessee to visit her mother in Louisiana.

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Media witnesses said Grayson cursed at a prison warden when asked if he had a final statement and made an obscene gesture.

They said Grayson shook his head from side to side when the gas began to flow into the mask over his face. He gasped for several minutes before he stopped moving and was pronounced dead.

Grayson, who was 19 at the time, and three other teenagers offered Deblieux a ride but took her instead to a wooded area where they beat her to death and mutilated her body, according to court documents.

She was stabbed 180 times, one of her lungs had been removed, and her fingers and thumbs were cut off.

"Over 30 years ago, Grayson and his accomplices brutally murdered a complete stranger and mutilated her body," Marshall's statement read. "It takes a truly vicious monster to commit this kind of crime."

Grayson was sentenced to death, while the other three participants in the murder, who were under the age of 18 at the time of the crime, are serving life sentences.

Alabama carried out the first execution in the US using nitrogen gas in January and performed a second one in September.

The execution is carried out by pumping nitrogen gas into a facemask, causing the prisoner to suffocate.

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Grayson chose nitrogen gas for his method of execution instead of lethal injection.

United Nations experts urged the Alabama authorities on Wednesday to halt the execution.

"This method may amount to cruel, inhuman or degrading treatment or even torture," the eight independent UN rights experts said in a joint statement.

"We reiterate our call for an urgent ban on execution by nitrogen asphyxiation, which is clearly prohibited under international law," the experts, who were appointed by the UN Human Rights Council but who do not speak on behalf of the United Nations, said.

More broadly, they called on the US to "join a growing global consensus towards universal abolition of the death penalty, beginning by promptly imposing a moratorium on executions."

The death penalty has been abolished in 23 states, while six others – Arizona, California, Ohio, Oregon, Pennsylvania, and Tennessee – have moratoriums in place. There have been 21 executions in the US this year.