The state also promised more to come in Alabama: Attorney General Steve Marshall said that 43 other people on death row had chosen asphyxiation over lethal injections since lawmakers approved the method in 2018.

Alabama called the new method "humane," while human rights groups condemned it as cruel and torturous.

"Alabama has done it, and now so can you, and we stand ready to assist you in implementing this method in your states," Marshall, a Republican, told reporters on Friday. Oklahoma and Mississippi lawmakers have also added nitrogen asphyxiation to their states' execution methods, but have not yet used it.

Marshall said asphyxiation by nitrogen, the first new execution method since lethal injections began in the U.S. in 1982, is "no longer an untested method."

"It is a proven one," he said.

There were diverging accounts as to how violent the method was between state officials and some who witnessed the public execution of Smith, who, unusually, survived a first execution attempt in 2022 when executioners struggled to insert an intravenous line for a lethal injection.

Alabama had predicted in court filings that, under its new method, Smith would slip into unconsciousness within about 30 seconds and die soon after. Executioners strapped a commercial industrial-safety respirator mask, made by a Canadian-owned safety product manufacturer called Allegro Industries, over the man's face and connected it to a canister of pure nitrogen.

Five journalists allowed to watch the execution through a window as media witnesses said he remained conscious for several minutes after the nitrogen flowed, and then began shaking and writhing on the gurney for about two minutes.

The Rev. Jeff Hood, who stood besides Smith as his spiritual adviser after signing a safety waiver acknowledging the risks of nitrogen asphyxiation, said Smith repeatedly threw his head forward as he struggled for life.

Alabama officials said everything went as expected. They said Smith appeared to hold his breath for as long as he could, and suggested the writhing could have been "involuntary movements."

"What occurred last night was textbook," Marshall said.

Alabama's Department of Forensic Sciences will perform an autopsy on Smith's body, prison officials said.

Smith was convicted of murdering Elizabeth Sennett after accepting $1,000 to kill her with accomplices at the behest of her husband, a preacher who later killed himself.

The jury voted 11-1 to sentence him to life in prison but an Alabama judge overruled their decision under a law that was later scrapped and ruled unconstitutional. Some of Sennett's relatives witnessed the execution, and told reporters afterwards that they had forgiven her killers and were glad the execution was over.

The American Civil Liberties Union, Amnesty International USA and other rights groups condemned the execution.

"The whole purpose of these methods is to hide pain," said Maya Foa, joint executive director of the rights advocacy group Reprieve. "How many more prisoners must die agonizing deaths before we see executions for what they really are: the state violently taking a human life?"

Smith unsuccessfully challenged Alabama's attempts to kill him in federal courts, arguing the botched first effort and the untested new method violated a constitutional ban on "cruel and unusual punishment."

Robert Grass, a lawyer who represented Smith in those challenges, said he was saddened by Smith's death, and described him as finding sobriety in prison and dedicating himself to help other prisoners get or stay sober.

"He found and sincerely practiced his faith," Grass said. "He studied and earned an associate's degree. He developed his artistic skill and would have made a very fine lawyer under different circumstances."

(Reporting by Jonathan Allen in New York; editing by Jonathan Oatis)

By Jonathan Allen