ATMORE, Ala. — Alabama is poised to use nitrogen gas in a planned execution next month, the first state to attempt such a method, setting the stage for legal challenges as officials across the U.S. examine alternatives amid a shortage of lethal injection drugs.

But while Alabama is intent on using nitrogen hypoxia, in which a person breathes only nitrogen and dies from a lack of oxygen, some details of the protocol remain cloaked in mystery to the public.

Even the inmate who is set to die, Kenneth Eugene Smith, told NBC News this month that he is not privy to an unredacted state protocol describing how the procedure will work. His legal and medical representatives were permitted this month to tour the execution chamber and inspect a mask for breathing the nitrogen, but without Smith.

“As goes Kenny, so goes the rest of my brothers,” Smith said of the 163 other death row inmates in the state during a phone interview from the William C. Holman Correctional Facility in Atmore.

Adding to the novelty of his case, Smith, 58, is a rare example of a person surviving a failed execution attempt: A previous plan to put him to death by lethal injection in November 2022 was called off when prison staff was unable to find a suitable vein. This, in addition to mounting scrutiny over the use of the lethal injection in other inmates, set off a pause in executions in Alabama.

“It’s not a one size fits all,” Smith said.

Smith was convicted for his role in the 1988 murder-for-hire slaying of a preacher’s wife, Elizabeth Sennett, in Alabama’s Colbert County. Her family said they are putting their faith in officials to complete the execution — scheduled for Jan. 25 — following decades of legal-related delays, but stopped short of saying they trust the state to carry out Smith’s sentence after the failed attempt last year.

“I mean, you can’t really test it on nobody, but I just hope they get it right this time,” Elizabeth Sennett’s son, Michael Sennett, said.

Lethal injection remains the primary method of executions in Alabama, but the state, which approved the use of nitrogen in 2018, wants to employ it in Smith’s case. When Smith’s execution failed last year, the state agreed they would not try to execute him again via lethal injection.

With so many lingering questions and the state’s dubious track record, death penalty experts and critics say the decision to try an untested method doesn’t instill public confidence.

Oklahoma and Mississippi have also approved the use of nitrogen hypoxia, but neither have tried it.

“It’s a slapdash thing,” Deborah Denno, a professor at Fordham Law School who studies the death penalty, said, adding that “the states are so desperate to keep executing people that they come up with a method and say, ‘this is foolproof,’ and then provide so few details.”

What the state protocol says — and doesn’t say

The public got a first glimpse of how an execution by nitrogen hypoxia could work when a protocol produced by the Alabama Department of Corrections was included in a court filing in August.

The inmate will be placed on a gurney in the execution chamber and given a pulse oximeter, which measures oxygen levels in the blood, the document says. A mask — described by officials in an affidavit as “used for industrial purposes” — will be tested to ensure “breathing air is being supplied” before it is strapped onto the inmate’s face.

A spiritual adviser will be permitted to enter the chamber and interact with the inmate as part of a previously approved plan. (The spiritual adviser will remain in the chamber until the execution is complete.)

Then, curtains to the chamber will be drawn so the execution can be viewed by witnesses. The inmate will be allowed to make a final statement, and if no stay of execution is granted, the mask will be inspected a final time and “the warden will activate the nitrogen hypoxia system,” according to the document.

Nitrogen gas would be administered into the mask for either 15 minutes or “five minutes following a flatline indication on the EKG, whichever is longer,” the document says.

But the outline of the procedure also includes heavily redacted passages related to how the oxygen-monitoring equipment is calibrated, how the nitrogen hypoxia system is operated, including various safety requirements, and the shutdown of the system.

Other details, including who is supplying the nitrogen gas and the mask, how the gas is generally being stored and what safety training the prison staff has undergone, remains unclear.

“All the parts we would need to know most about are redacted,” said Denno, the Fordham professor. “Where are they getting the gas? That’s very important. Even if you don’t want to mention the manufacturer, you want to know, as we do with our lethal injection drugs: Is the gas coming from a legitimate source? Is it being delivered to the Department of Corrections or is it being made somewhere?”

The Alabama attorney general’s office did not immediately respond to a request for comment regarding further details about the nitrogen gas method. A source familiar with Attorney General Steve Marshall’s thinking said he is “determined” to see Smith’s execution through, telling NBC News that he has been pushing for the development of the nitrogen protocol during his time in office, describing it as a “non-negotiable” before his term ends in January 2027.

“This new method may prove to be more efficient for the government’s use and has the tangential benefit of being painless,” said the source, referring to challenges with lethal injection. “Mrs. Sennett’s death, of course, was not.”

In a prior statement, Marshall said that it is necessary for the execution of Smith to move forward.

“Though the wait has been far too long, I am grateful that our talented capital litigators have nearly gotten this case to the finish line,” Marshall said.

How does nitrogen hypoxia kill?

Nitrogen is a naturally occurring odorless and colorless gas; it is abundant — found in the earth’s atmosphere and the soil — and it can be used to rapidly freeze food in its liquid form.

But if not mixed with an appropriate amount of oxygen, breathing it in can lead to adverse physiological effects, such as abnormal fatigue, impaired respiration, vomiting and even death.

Joel Zivot, a practicing physician in anesthesiology and intensive care medicine at Emory University in Atlanta, said he would like to know how Alabama prison officials plan to deliver pure nitrogen without allowing oxygen to mix with it.

Read Related Also: Seniors using AI robots to combat loneliness, use for companionship

If “nitrogen is mixed with any amount of oxygen, any amount of air, then it will take longer for the nitrogen to cause death, or it may never cause death,” said Zivot, adding that if the process is prolonged, “you’re talking about basically death by slow asphyxiation, which the body will interpret as profoundly uncomfortable and frightening.”

“There is no room for error,” he said.

An unintended nitrogen leak can also have disastrous consequences, Zivot warned.

“The thing about lethal injection is that once the catheter is in and the medication is being injected, there really should be no risk to anybody, any observer or anybody in the room because the chemicals don’t leave the body of the prisoner and touch anyone else,” he said. “But of course, gas will go wherever it wants to go, wherever there are places for it to go.”

A U.S. Chemical Safety Board report that reviewed dozens of cases of nitrogen asphyxiation between 1992 and 2002 found 80 deaths, with the majority of incidents occurring in manufacturing and industrial settings. The board said causes of death included a failure to detect the lack of enough oxygen in a confined space and a mistaken use of nitrogen gas instead of breathing air.

In 2021, six workers died of nitrogen asphyxiation from a liquid nitrogen leak at a chicken processing plant in Georgia.

At a hearing this month, Smith’s legal team argued before a federal judge that the use of nitrogen hypoxia would violate the constitutional ban on cruel and unusual punishment. An anesthesiologist testifying on behalf of Smith said he could vomit with a risk that he chokes, experience the sensation of feeling suffocated or potentially be left in a vegetative state.

State prosecutors called those possibilities speculative and said they believe it’s possible for nitrogen to be used humanely.

The American Veterinary Medical Association has affirmed nitrogen hypoxia for euthanizing only certain animals, such as chickens and turkeys, and recommends giving larger animals a sedative. Understanding how human bodies react to nitrogen hypoxia is limited because it is not something medical professionals can legitimately study as it relates to executions; it’s unclear if Alabama’s protocol recommends the use of a sedative.

The American Medical Association “cannot ask physicians to violate professional ethics by imparting medical knowledge that can aid or contribute to the ability of others to carry out methods of capital punishment,” a spokesperson said.

Could the execution be delayed?

U.S. District Judge R. Austin Huffaker Jr. did not indicate when he may rule on Smith’s case, but in a court order last week suggested that Alabama make at least one compromise by allowing his last rites be given before the mask is secured.

Smith’s spiritual adviser, the Rev. Jeff Hood, filed a lawsuit this month claiming the state is violating his religious liberties by requiring him to be no closer than 3 feet to Smith in the execution chamber.

Hood, an opponent of the death penalty, said Alabama has since agreed to allow him to be with Smith before his mask is affixed and that he can touch Smith during the execution. He still has questions about his safety.

“If a nitrogen leak were to happen in the execution chamber, who will be there?” Hood asked during a news conference after filing his complaint. “What is the emergency procedure if I collapse?”

The Alabama Department of Corrections declined to comment on pending litigation.

Smith was 22 when pastor Charles Sennett hired him and two others for $1,000 each to kill his wife so he could collect on her life insurance, prosecutors said. Elizabeth Sennett, 45, was stabbed and beaten to death in her home.

“If I could turn back the hand of time I’d absolutely do it,” Smith said this month. “There’s been many lives lost already from this case, and this is going to make more victims.”

Sennett’s family disagrees.

“It doesn’t matter to me how he goes out, so long as he goes,” Michael Sennett said.