Alabama told a federal judge this week that it has finished construction of a “system” to use nitrogen gas to carry out death sentences, an execution method authorized by state law but never put into use.

The Alabama Department of Corrections indicated in an Aug. 2 court filing that it is waiting to make sure the nitrogen hypoxia system is ready, before writing procedures for how it will be used. The prison system did not describe how the system would work or give an estimate on when the state may try to use the new execution method.

“The ADOC has completed the initial physical build on the nitrogen hypoxia system. A safety expert has made a site visit to evaluate the system. As a result of the visit, the ADOC is considering additional health and safety measures,” a lawyer for the state attorney general’s office wrote in the court filing.

Alabama in 2018 became the third state — along with Oklahoma and Mississippi — to authorize the untested use of nitrogen gas to execute prisoners.

Death would be caused by forcing the inmate to breathe only nitrogen, thereby depriving him or her of oxygen. Lawmakers theorized that death by nitrogen hypoxia could be a simpler and more humane execution method. But critics have likened the untested method to human experimentation.

No state has used nitrogen hypoxia to carry out an execution, and no state has developed a protocol for its use, according to the Death Penalty Information Center.

The information of the construction was disclosed in a court filing involving a lawsuit over the presence of spiritual advisers in the death chamber. State lawyers wrote that they did not yet know if a spiritual advisers could safely be present during an execution via nitrogen hypoxia.

A prison system spokesman did not immediately respond to an email seeking comment. The prison system has previously declined to describe the proposed system, citing security concerns.

Alabama currently carries out executions by lethal injections unless an inmate requests the electric chair. As lethal injection drugs become difficult to obtain, states have begun looking at alternative ideas for carrying out death sentences including firing squads and gas.