There are no reports of nitrogen gas ever being used to execute humans, and critics say that one concern is that the method is untested. Some states even ban its use to put animals to sleep.

"I support that policy, and I believe capital punishment must be performed effectively and without cruelty," Governor Mary Fallin said. "The bill I signed today gives the state of Oklahoma another death penalty option that meets that standard."

The bill easily passed a Republican-controlled state Legislature. Nitrogen will provide an alternative method if lethal injections aren't possible, either because of a court ruling or a drug shortage.

The changes come after a botched execution last year in which Oklahoma was using a new sedative as the first in a three-drug combination. Officials tried to halt the lethal injection after the inmate writhed and moaned. He died 43 minutes after the process began.

Supporters say nitrogen-induced hypoxia is humane. "The process is fast and painless," said Mike Christian, a former Oklahoma Highway Patrol trooper who wrote the bill. "It's foolproof."

Opponents say there's no way to know whether it is painless and effective. "It just hasn't been tried, so we don't know," said Emily Virgin, a Democrat who opposes the death penalty.

Last year's problematic execution was blamed on a poorly placed intravenous line and prompted a lawsuit from Oklahoma death row inmates, who argue that the state's new drug combination presents a serious risk of pain and suffering. The US Supreme Court is scheduled to hear arguments later this month.