ATMORE, Ala. (NewsNation) — Witnesses to Alabama’s nitrogen gas execution say convicted murderer Kenneth Eugene Smith appeared to shake and writhe on the gurney for at least two minutes, sometimes pulling against restraints.

The execution took about 22 minutes, and Smith appeared to remain conscious for several minutes. Some critics to the first-of-its-kind method have called it inhumane, cruel and experimental.

“This is a homicide,” forensic analyst Joseph Scott Morgan said in an interview Thursday night on “Banfield” after the execution occurred.

“It should be investigated as (a homicide),” Morgan said. “This is the same as an in-custody death with a police officer. They need to thoroughly investigate in the autopsy room.”

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 in 2022, but the lethal injection was called off at the last minute because of difficulty connecting an IV line.

The execution came after a last-minute legal battle in which his attorneys contended the state was making Smith 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.

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 justices in the majority did not issue any written opinion providing a reasoning for denying the appeal.

The state had predicted the nitrogen gas would cause unconsciousness within seconds and death within minutes. State Attorney General Steve Marshall said late Thursday that nitrogen gas “was intended to be — and has now proved to be — an effective and humane method of execution.”

Asked about Smith’s shaking and convulsing on the gurney, Alabama Department of Corrections Commissioner John Q. Hamm said they appeared to be involuntary movements.

“That was all expected and was in the side effects that we’ve seen or researched on nitrogen hypoxia,” Hamm said. “Nothing was out of the ordinary from what we were expecting.”

The Associated Press contributed to this report.