Alabama executed a convicted murderer with nitrogen gas Thursday, putting him to death with a first-of-its-kind method that once again placed the U.S. 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 8:25 p.m. 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 from the time between the opening and closing of the curtains to the viewing room. 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. Alabama Gov. 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,” Ivey said in a statement. “I pray that Elizabeth Sennett’s family can receive closure after all these years dealing with that great loss.”

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,” he said. “I’ll end by saying Elizabeth Dorlene Sennett got her justice tonight.”

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 U.S. Supreme Court.