ATMORE, Alabama, Jan 25 (Reuters) - Alabama on Thursday executed convicted murderer Kenneth Smith, who held his breath in vain as officials asphyxiated him with nitrogen gas, the first use of a new method of capital punishment since lethal injections began in the U.S. four decades ago.

Smith, convicted of a 1988 murder-for-hire, was a rare prisoner who had already survived one execution attempt. In November 2022, Alabama officials aborted his execution by lethal injection after struggling for hours to insert an intravenous line's needle in his body.

The state has called its new closely watched protocol "the most painless and humane method of execution known to man." It predicted Smith would lose consciousness in under a minute and die soon after, although witnesses on Thursday said it appeared to take several minutes longer.

Alabama has touted asphyxiation as a simpler alternative for prison systems that struggle to find either veins or the required drugs for lethal injections.

Human rights groups, United Nations torture experts and lawyers for Smith had sought to prevent it, saying the method was risky, experimental and could lead to an agonizing death or non-fatal injury.

In Smith's second and final trip to the execution chamber on Thursday, executioners restrained him in a gurney and strapped a commercial industrial-safety respirator mask to his face. A canister of pure nitrogen was attached to the mask that, once flowing, deprived him of oxygen.

The execution began at 7:53 p.m. (0153 GMT Friday) and Smith was declared dead at 8:25 p.m. (0225 GMT), prison officials said.

Smith appeared to remain conscious for several minutes after the nitrogen was activated, according to five journalists who were allowed to watch the execution through glass as media witnesses. Although the mask was also secured to the gurney, he then began shaking his head and writhing for about two minutes, and then could be seen breathing deeply for several minutes before his breathing slowed and became imperceptible, the witnesses said.

"It appeared that Smith was holding his breath as long as he could," Alabama Corrections Commissioner John Hamm told a press conference. "He struggled against the restraints a little bit but it's an involuntary movement and some agonal breathing. So that was all expected."

Rev. Jeff Hood, Smith's spiritual adviser, was at Smith's side for the execution, and said prison officials in the room "were visibly surprised at how bad this thing went."

"What we saw was minutes of someone struggling for their life," Hood, attending his fifth execution in the last 15 months, told reporters.

"We saw minutes of someone heaving back and forth. We saw spit. We saw all sorts of stuff from his mouth develop on the mask. We saw this mask tied to the gurney, and him ripping his head forward over and over and over again."

Before the nitrogen was switched on, Smith made a lengthy final statement that began: "Tonight, Alabama caused humanity to take a step backward."

His wife and other relatives attended and he gestured towards them. "I'm leaving with love, peace and light," he said, according to media witnesses. "Love all of you."

Photo from Reuters