Alabama has carried out the execution of a convicted murderer using nitrogen gas, employing a method that is unprecedented and has once again sparked a contentious debate surrounding capital punishment in the United States. The state asserted that the method would ensure humane execution, yet critics condemned it as both cruel and experimental.

Kenneth Eugene Smith, aged 58, was declared deceased at 8:25 pm Thursday (0225 GMT Friday) at an Alabama prison after inhaling pure nitrogen gas via a face mask, leading to oxygen deprivation. This marks the inaugural utilization of a new execution technique in the United States since the introduction of lethal injection, which has been the predominant method since 1982.

The execution process lasted approximately 22 minutes, during which Smith seemed to maintain consciousness for several minutes.

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.

Alabama Governor Kay Ivey said the execution was justice for the murder-for-hire killing of 45-year-old Elizabeth Sennett in 1988.

“After more than 30 years and attempt after attempt to game the system, Mr Smith has answered for his horrendous crimes … I pray that Elizabeth Sennett’s family can receive closure after all these years dealing with that great loss,” Ivey said in a statement.

Mike Sennett, the victim’s son, said Thursday night that Smith “had been incarcerated almost twice as long as I knew my mom.”

“Nothing happened here today is going to bring Mom back. It’s kind of a bittersweet day. We are not going to be jumping around, whooping and holler, hooray and all that … I’ll end by saying Elizabeth Dorlene Sennett got her justice tonight,” he said.

The state had previously attempted to execute Smith 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 his attorneys contended the state was making him the test subject for an experimental execution method that could violate the constitutional ban on cruel and unusual punishment.

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

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.