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Oklahoma Gov. Mary Fallin has signed a law making an untested execution method — nitrogen gas — the backup to lethal injections. The nitrogen, which starves the body of oxygen, replaced the electric chair as the alternate to the needle.

"Oklahoma executes murderers whose crimes are especially heinous," Fallin said in a statement. "I support that policy, and I believe capital punishment must be performed effectively and without cruelty. The bill I signed today gives the state of Oklahoma another death penalty option that meets that standard."

Oklahoma's executions are on hold while the U.S. Supreme Court decides whether its lethal injection combination — specifically the use of the controversial drug midazolam — is constitutional or too cruel to use on condemned murderers.

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It was a badly botched execution in Oklahoma last April — when Clayton Lockett regained consciousness during his lethal injection — that reignited the national debate over capital punishment and unleashed a flood of lawsuits.

Because nitrogen hypoxia has not been used as a form of execution in the U.S., it is certain to trigger legal challenges.

Fordham Law professor Deborah Denno, an expert on capital punishment, said that while proponents insist the method is humane and painless there is scant detail on who would carry it out and how.

"As far as I know there also appears to be no medical or science expert involved with the method’s design or details," she said. "All of these questions are critical to address before any serious steps are taken in light of this country’s history of botched executions and ignorance of the methods adopted."

Oklahoma is one of several states that have turned to alternate execution methods after running into trouble obtaining lethal injection chemicals because big pharmaceutical companies won't sell them to prisons any more.

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