ALABAMA HAS EXECUTED a convicted murderer with nitrogen gas, putting him to death with a new method that once again put the US at the forefront of the debate over capital punishment.

The state said the method would be humane, but critics called it cruel and experimental.

Officials said Kenneth Eugene Smith, 58, was pronounced dead at 8:25pm at an Alabama prison after breathing pure nitrogen gas through a face mask to cause oxygen deprivation.

It marked the first time that a new execution method has been used in the United States since lethal injection, now the most commonly used method, was introduced in 1982.

The execution took about 22 minutes, and Smith appeared to remain conscious for several minutes.

For at least two minutes, he appeared to shake and writhe on the gurney, sometimes pulling against the restraints. That was followed by several minutes of heavy breathing, until breathing was no longer perceptible.

In a final statement, Smith said: “Tonight Alabama causes humanity to take a step backwards. … I’m leaving with love, peace and light.”

He made the “I love you sign” with his hands toward family members who were witnesses. “Thank you for supporting me. Love, love all of you,” Smith said.

The state had previously attempted to execute Smith, who was convicted of a 1988 murder-for-hire, in 2022, but the lethal injection was called off at the last minute because authorities couldn’t connect an IV line.

The execution came after a last-minute legal battle in which Smith’s attorneys contended the state was making him the test subject for an experimental execution method.

Federal courts rejected Smith’s bid to block it, with the latest ruling coming Thursday night from the US Supreme Court.

Anti-death penalty activists place signs along the road heading to Holman Correctional Facility Alabama ahead of the scheduled execution of Kenneth Eugene Smith AP AP

Justice Sonia Sotomayor, who along with two other liberal justices dissented, wrote: “Having failed to kill Smith on its first attempt, Alabama has selected him as its ‘guinea pig’ to test a method of execution never attempted before. The world is watching.”

The majority justices did not issue any statements.

In a statement issued before he was put to death, Smith and the reverend Jeff Hood, his spiritual adviser, said, “The eyes of the world are on this impending moral apocalypse. Our prayer is that people will not turn their heads. We simply cannot normalize the suffocation of each other.”

The state had predicted the nitrogen gas would cause unconsciousness within seconds and death within minutes.

A state attorney told the 11th circuit court of appeals that it will be “the most painless and humane method of execution known to man.”

But some doctors and organisations have raised alarm, and Smith’s attorneys had asked the Supreme Court to halt the execution to review claims that the method violates the constitutional ban on cruel and unusual punishment and deserves more legal scrutiny before it is used on a person.

“There is little research regarding death by nitrogen hypoxia. When the State is considering using a novel form of execution that has never been attempted anywhere, the public has an interest in ensuring the State has researched the method adequately and established procedures to minimize the pain and suffering of the condemned person,” Smith’s attorneys wrote.

In her dissent, Sotomayor wrote that Alabama has shrouded its execution protocol in secrecy, releasing only a heavily redacted version.

She also said Smith should be allowed to obtain evidence about the execution protocol and to proceed with his legal challenge.

“That information is important not only to Smith, who has an extra reason to fear the gurney, but to anyone the State seeks to execute after him using this novel method,” Sotomayor wrote.

“Twice now this Court has ignored Smith’s warning that Alabama will subject him to an unconstitutional risk of pain,” she wrote. “I sincerely hope that he is not proven correct a second time.”

Justice Elena Kagan wrote a separate dissent and was joined by justice Ketanji Brown Jackson.

In his final hours, Smith met with family members and his spiritual adviser, according to a prison spokesperson.

He ate a last meal of T-bone steak, hash browns, toast and eggs slathered in A1 steak sauce, reverend Hood said by telephone before the execution was carried out.