MONTGOMERY, Ala. (AP) — Alabama is seeking to become the first state to execute a prisoner by making him breathe pure nitrogen.

The Alabama attorney general’s office on Friday asked the state Supreme Court to set an execution date for death row inmate Kenneth Smith. Alabama plans to put him to death by nitrogen hypoxia, an execution method that is authorized in three states but has never been used.

Nitrogen hypoxia is caused by forcing the inmate to breathe only nitrogen, depriving them of oxygen and causing them to pass out and die, according to the theory. Nitrogen makes up 78% of the air inhaled by humans and is harmless when inhaled with oxygen.

Critics have likened the untested method to human experimentation.

Alabama authorized nitrogen hypoxia in 2018 but the state has not attempted to use it until now to carry out a death sentence. Oklahoma and Mississippi have also authorized nitrogen hypoxia.

Alabama has been working for several years to develop the execution method, but has disclosed little about the proposal. The attorney general’s court filing did not disclose the details of the how the execution would be carried out. Corrections Commissioner John Hamm told reporters last month that a protocol was nearly complete.

Smith’s execution by lethal injection was called off last year because of problems with intravenous lines. Smith was convicted in the 1988 murder-for-hire slaying of a preacher’s wife.

Prosecutors said Smith was one of two men who were each paid $1,000 to kill Elizabeth Sennett on behalf of her husband, who was deeply in debt and wanted to collect on insurance. The slaying, and the revelations over who was behind it, rocked the small north Alabama community.

A number of Alabama inmates, including Smith, in seeking to block their executions by lethal injection, have argued they should be allowed to die by nitrogen hypoxia. The disclosure that the state is ready to use nitrogen hypoxia is expected to set off a new round of legal battles over the constitutionality of the method.

“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,” Alabama Attorney General Steve Marshall said in a statement.