Alabama told a federal judge on Tuesday that it has nearly finished construction to use nitrogen gas to carry out death sentences, an execution method authorized by state law but never put into use.

The court filing did not describe how the proposed execution system would work. When the Alabama legislation was approved authorizing nitrogen hypoxia, proponents theorized that death by nitrogen hypoxia could be a simpler and more humane execution method. Death would be caused by forcing the inmate to breathe only nitrogen, thereby depriving him or her of oxygen.

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

Lawyers for the state wrote in a court filing Tuesday that the Alabama Department of Corrections “is nearing completion of the initial physical build for the nitrogen hypoxia system and its safety measures.”

“Once the build is completed, a safety expert will make a site visit to evaluate the system and look for any points of concern that need to be addressed,” the lawyers added.

The information was disclosed in a court filing Tuesday 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.

The prison system declined to describe the proposed system. The Alabama Department of Corrections wrote in an email to The Associated Press that the, “nitrogen hypoxia execution protocol is still under development, and the physical building modifications to the execution chamber are still in process.”

“Due to the fact those two items are not yet in a finalized state and potential security concerns exist, that is all we are able to share at this time,” a department spokeswoman wrote.

The department of corrections did not give an estimate in the court filing on when the state might try to use nitrogen hypoxia. The prison system said it had not yet drafted a protocol for using nitrogen gas.

But lawyers for the state wrote in Tuesday’s filing that a protocol would not be drawn up until authorities were satisfied the system was ready for use.

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.

No state has used nitrogen hypoxia to carry out an execution and no state has developed a protocol for its use, said Robert Dunham, executive director of the Death Penalty Information Center. He likened the planned first usage to human experimentation.

“It’s almost by definition an experimental process,” Dunham said.