Alabama has executed a convicted murderer with nitrogen gas, putting him to death with a new method the US state said would be humane but critics called cruel and experimental.
Kenneth Eugene Smith, 58, was pronounced dead at 8.25pm on Thursday (1.25pm on Friday AEDT) at the Holman Correctional Facility in Atmore, officials said, 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 came after a last-minute legal battle in which Smith's lawyers 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 the execution, and it was delayed for two hours as the US Supreme Court weighed his final appeal and ultimately allowed the death penalty to proceed.
Alabama prison officials and journalists who observed Smith's execution were due to brief reporters later on Thursday evening.
Alabama called its new protocol "the most painless and humane method of execution known to man".
United Nations human rights experts and lawyers for Smith, who survived Alabama's previous botched attempt to execute him by lethal injection, had sought to prevent it, saying the method was risky, experimental and could lead to a torturous death or non-fatal injury.
Smith, convicted of a 1988 murder-for-hire, was a rare prisoner who has 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.
"On March 18, 1988, 45-year-old Elizabeth Sennett's life was brutally taken from her by Kenneth Eugene Smith," Governor Ivery said in a statement.
"After more than 30 years and attempt after attempt to game the system, Mr Smith has answered for his horrendous crimes."
Though poisonous gases such as hydrogen cyanide have been used in executions, this was the first time a death sentence had been carried out anywhere using an inert gas to suffocate someone, capital punishment experts said.
"It's a sad, awful day for everyone, no matter what your perspective is," Reverend Jeff Hood, Smith's spiritual adviser, said before heading into the prison earlier on Thursday.
"But I think that this is particularly horrific in that we're going to be conducting a human experiment for the first time. We're going to be legally suffocating someone."
Smith's lawyers feared the mask will not properly seal against Smith's face, allowing oxygen to seep in, delaying or even averting the moment of unconsciousness but risking serious brain injury.
Australian euthanasia expert Dr Philip Nitschke, who appeared as a witness for Smith, said nitrogen could provide a peaceful, hypoxic death, but leaks in the mask could result in Smith drawing in enough oxygen "to prolong into what could be a very rather macabre, slow process of slowly not getting enough oxygen”.
Some states are looking for new ways to execute people because the drugs used in lethal injections have become difficult to find.
Alabama, Mississippi and Oklahoma have authorised nitrogen hypoxia as an execution method, but no state has attempted to use the untested method until now.
Smith was convicted of murdering Elizabeth Sennett, a preacher's wife, in 1988 after he and an accomplice each accepted a $US1000 fee from her husband to kill her, according to trial testimony.
Reuters with AP
