Alabama will become the third state to authorize the untested use of nitrogen gas to execute prisoners under legislation signed by Gov. Kay Ivey.

Ivey signed the bill Thursday, two days after it was approved by lawmakers.

Alabama currently carries out executions by lethal injection. The bill would allow execution by nitrogen hypoxia if lethal injection drugs are unavailable or ruled unconstitutional. Breathing the inert gas causes oxygen depletion in the blood stream.

The Death Penalty Information Center says Oklahoma and Mississippi allow execution by nitrogen gas, but no state has used the method. Oklahoma announced last week it will begin using nitrogen because of difficulty obtaining lethal injection drugs.

Alabama lawmakers argued that the method could be humane. Opponents noted that it has never been tested.

(AP)