By Dakin Andone, CNN

A federal judge has granted an order prohibiting Alabama from executing a death row inmate by lethal injection this week after he asserted he chose to die by nitrogen hypoxia -- an untested and unproven execution method Alabama officials say they're not ready to use.

Without the injunction, the inmate, Alan Eugene Miller, would "likely suffer irreparable injury," US District Judge Austin Huffaker Jr. wrote in his order, "because he will be deprived of the ability to die by the method he chose and instead will be forced to die by a method he sought to avoid and which he asserts will be painful."

As a result, the state cannot execute Miller "by any method other than nitrogen hypoxia until further order from the court."

The case has put a spotlight on nitrogen hypoxia, which experts and critics say has yet to be proven humane or effective and could never be ethically tested, despite proponents' claims it could be simpler, easier and safer than lethal injection. Inmates like Miller, however, are making an "uninformed choice," said Robert Dunham, executive director of the Death Penalty Information Center, because the method has never been used.

The judge's order Monday comes after Miller -- sentenced to death for a 1999 triple killing -- sued the commissioner of the Alabama Department of Corrections, the state attorney general and his warden, alleging corrections officials were moving to execute him by lethal injection after losing paperwork in which he claimed to have chosen to die by breathing nitrogen gas.

The failure to honor his request, Miller's complaint said, violated his constitutional rights.

State officials suggested Miller made no such choice and said they had no record of his preference, court filings show.

The case attracted widespread attention after an attorney for the Corrections Department last week said it was "very likely" the state could carry out Miller's execution by nitrogen hypoxia, which in theory involves replacing the critical oxygen in the air with nitrogen until the inmate is breathing 100% nitrogen.

The declaration was notable -- albeit "vague and imprecise," per the judge -- coming more than four years after Alabama authorized the method as an alternative to lethal injection, which remains the primary execution method for the US federal government and the 27 states that still have capital punishment.

Only two other states -- Oklahoma and Mississippi -- have approved use of nitrogen hypoxia to carry out death sentences, but none has ever actually used it.

Days after Alabama made the claim, though, its corrections chief walked it back. The state "cannot carry out an execution by nitrogen hypoxia," Commissioner John Hamm wrote in a court filing, but remained "ready to carry out Plaintiff's sentence by lethal injection on September 22, 2022."

That led to the judge's order Monday, which appears to force the state to iron out its nitrogen hypoxia execution protocol before moving forward with Miller's execution. It's unclear when that might be ready.

"Suffice it to say," the judge wrote, "the readiness of the protocol and of the (Corrections Department) to conduct executions by nitrogen hypoxia has been a moving target."

The department had "completed many of the preparations necessary for conducting executions by nitrogen hypoxia," but its protocol was "not yet complete," it told CNN last week in a statement. "Once the nitrogen hypoxia protocol is complete, (department) personnel will need sufficient time to be thoroughly trained before an execution can be conducted using this method."

Death by nitrogen gas

The claims of those who back executions by nitrogen gas might sound appealing, considering states' continued problems obtaining the drugs for lethal injections and with recent executions deemed botched, either because an inmate suffered inordinately or because the process deviated from officials' prescribed protocol.

But critics and experts reject those arguments, saying there is no proof executions by nitrogen hypoxia would adhere to inmates' constitutional protection against cruel and unusual punishment because it has never been used and could never be ethically tested.

"There could be no legitimate research. There's no way you could design a research project that would be ethical ... There will never be a human study. It has no medical reason to be conducted and would never pass any kind of ethical oversight that would permit such a thing to take place," said Dr. Joel Zivot, an associate professor of anesthesiology and surgery at Emory University who objects to use of the term "nitrogen hypoxia," preferring "execution with nitrogen gas."

"This is all just fake, guess. It's all unknown," he told CNN. "They can't test it ethically. They're just going to try it ... and then afterward they're going to decide whether or not they think they've killed someone and whether it's cruel or not."

And yet, inmates like Miller are opting for the unproven method due to concerns over the level of pain they might suffer during lethal injection, Dunham, of the Death Penalty Information Center, told CNN: "They are opting for a method that they hope will not be torturous over a method that they are certain will be torturous."

None of the three states that's approved nitrogen hypoxia as an execution method has yet to publicly release a protocol outlining how it would work. In addition to oxygen, about 78% of Earth's air is made up of nitrogen; in theory, an execution by nitrogen gas would involve replacing the oxygen in the air with nitrogen until the inmate suffocates in what advocates describe as a painless death.

The gas could be delivered in a chamber or through a mask, said Dunham, whose organization describes itself as one that does not take a stance on capital punishment but has been critical of the way it is administered.

The Alabama state lawmaker who sponsored the 2018 bill to authorize nitrogen hypoxia as an execution method believed it would be more humane than lethal injection, he told AL.com at the time. Oklahoma officials made similar statements that same year when they announced their intent to move forward with executions by nitrogen hypoxia or inert gas inhalation.

The week before the announcement, Alabama had aborted the execution of Doyle Hamm after he was punctured 11 times in a failed effort to set an intravenous line. That, then-director of the Oklahoma Department of Corrections Joe Allbaugh said, was "inhumane."

With the new methods, "after a couple of breaths, the individual loses consciousness, doesn't feel anything," he claimed.

But that is speculation, Zivot said.

While there are known cases of people dying by breathing too much nitrogen -- Zivot pointed to accidental deaths in industrial settings and suicides -- an execution is different, he said. It's impossible to know how it feels to die by breathing nitrogen gas, and it's not clear what such a death might look like or whether it would comport with the Eighth Amendment protection from "cruel and unusual punishments."

"Can nitrogen gas kill people? Yes," he said. "So can a big boulder ... I understand the chemistry of how nitrogen gas can kill people. But whether nitrogen gas can kill people and comport with the Constitution, that is not known."

'An uninformed choice'

Even without a full, official picture of what a nitrogen hypoxia execution should look like, Alabama inmates like Miller might choose it because of the apparent risks associated with lethal injection, Dunham said.

"The odds of being tortured to death by lethal injection are pretty substantial. The odds of a botch with nitrogen hypoxia are uncertain," Dunham told CNN. "I think it's a choice to avoid a sure bad thing, as opposed to an affirmative embrace of nitrogen hypoxia."

In his complaint, Miller and his attorneys pointed to two such Alabama cases: The first was Hamm's, and the second was the July execution of Joe Nathan James, which has been widely criticized, particularly after a report in The Atlantic said a private autopsy showed James' "suffered a long death."

The execution team "strictly followed the established protocol" for James' death, which was preceded by a three-hour delay, a spokesperson for the Corrections Department previously told the Montgomery Advertiser: "The protocol states that if the veins are such that intravenous access cannot be provided, the team will perform a central line procedure. Fortunately, this was not necessary and with adequate time, intravenous access was established."

Zivot, who told CNN he witnessed James' private autopsy, said the state "tortured him for three hours," cutting into his skin to try to find a vein to set an intravenous line -- which is not part of Alabama's lethal injection protocol.

It's executions like these death row inmates hope to avoid by choosing nitrogen hypoxia, Dunham said. Indeed, claims states have made about nitrogen hypoxia being fast, effective and "relatively painless," he said, echo the arguments states offered about the electric chair and lethal injection, which was first used by Texas in 1982.

"They believe it," Dunham said of nitrogen hypoxia's supporters. "There's no question about that. And they might be right. But they might also be wrong, and we won't know the answer until if and when executions by nitrogen suffocation take place."

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CNN's Jamiel Lynch contributed to this report.