Updated August 29, 2023 at 6:41 PM ET

Alabama's attorney general is asking the state's supreme court to allow the first-ever use of nitrogen hypoxia, a controversial and untested method, in an execution.

In a Friday court filing, Attorney General Steve Marshall requested the court to authorize a date for the execution of Kenneth Eugene Smith, who was convicted in 1996 in a murder for hire.

Smith's execution was previously scheduled for Nov. 17, 2022. However, he had opposed being executed by lethal injection, and petitioned in court for nitrogen hypoxia to be used instead.

Nitrogen hypoxia is a form of inert gas asphyxiation, in which a person is forced to breathe pure nitrogen until they die. While nitrogen makes up about 78% of the air in Earth's atmosphere, it must be mixed with sufficient amounts of oxygen for people to breathe it.

Back in November, the U.S. Supreme Court had overruled a stay of execution for Smith, and granted the state the ability to use lethal injection. But corrections officers could not complete the execution before the death warrant expired, due to problems inserting an IV into Smith's veins.

So the Supreme Court ruled in May that Smith's execution could go forth with nitrogen hypoxia.

"It is a travesty that Kenneth Smith has been able to avoid his death sentence for nearly 35 years after being convicted of the heinous murder-for-hire slaying of an innocent woman, Elizabeth Sennett," Marshall said in a statement about the execution date request.

Smith's attorney, Robert Grass, said he had no statement at this time.

Alabama would be the first state to perform an execution using nitrogen hypoxia. Two other states, Oklahoma and Mississippi, have approved the method for executions, but it still has not been used.

Alabama paused lethal injections a few days after the 2022 failed attempt on Smith – its third failed attempt since 2018. Lethal injections resumed in the state in July.

Likewise, there have been concerns about using nitrogen hypoxia, as it is a largely untested method.

The Supreme Court denied a Missouri inmate's request to be executed by the method in 2019. In the majority opinion, Justice Neil Gorsuch said it did not include information about how the nitrogen gas would be administered, in what amounts, how long it would take to work and how to keep the execution team safe from its effects.

Smith was convicted for a 1988 case in which Charles Sennett, a pastor in Alabama, paid Billy Gray Williams to kill his wife, so he could collect insurance money to settle debts. Williams then hired Smith and John Forrest Parker to carry out the murder, for which they each were supposed to be paid $1,000, according to prosecutors.

Smith was first convicted of capital murder and sentenced to death by a jury in 1989, but his sentence was appealed in 1992. In a second trial in 1996, a jury again found him guilty and he was sentenced to life in prison without the possibility of parole. However, a judge overruled the jury's decision and gave Smith the death penalty, according to the Montgomery Advertiser.

Copyright 2023 NPR. To see more, visit https://www.npr.org.