Gov. Kay Ivey has signed into law a bill to allow Alabama to carry out executions by nitrogen hypoxia, an alternative to lethal injection.

The governor signed the bill today, Ivey Press Secretary Daniel Sparkman said.

Alabama becomes the third state to authorize executions by lethal injection as a backup method, but the process has never been used, according to the Death Penalty Information Center.

Under the new Alabama law, condemned inmates could choose to die by nitrogen hypoxia instead of lethal injection. Or, if lethal injection was ruled unconstitutional or became otherwise unavailable, nitrogen hypoxia would become the state's method.

The law also still allows inmates to choose execution by electrocution, although none have done so since the state adopted lethal injection.

The nitrogen hypoxia bill was sponsored by Rep. Trip Pittman, R-Montrose. The House of Representatives passed it by a vote of 75-23 on Tuesday. It passed the Senate 29-0 in February.

Robert Dunham, executive director of the Death Penalty Information Center, said Oklahoma and Mississippi have authorized executions by nitrogen hypoxia. Dunham said the process has never been used and its unclear if it would be done in a vacuum chamber or with a death mask.

Dunham said Oklahoma is developing a protocol but has not released any specifics.

This story will be updated.