Facing dwindling supplies of lethal injection chemicals and increased legal scrutiny of the practice, some states are considering a return to antiquated execution methods like firing squads and gas chambers -- and Oklahoma is considering using a new type of gas. But experts warn the problem with both new and old methods is the same: They may violate the Constitution's ban on cruel and unusual punishment.

"States have painted themselves into a corner with lethal injection and are trying to bring back these old methods, Richard Dieter, executive director of the Death Penalty Information Center, a nonprofit that distributes information about capital punishment, told The Huffington Post Tuesday. There is no painless method.

Allegations of torture and cruel and unusual punishment surfaced in the wake of botched lethal injections last year, like those of Oklahoma inmate Clayton Lockett and Arizona inmate Joseph Wood . Last month, the U.S. Supreme Court delayed execution for three Oklahoma prisoners while it reviews the states protocol.

In response, Oklahoma legislators recently advanced bills that would authorize nitrogen hypoxia" -- which causes death by depleting the oxygen supply in the blood -- as a gas chamber alternative to poisonous hydrogen cyanide gas.

Rep. Mike Christian (R-Oklahoma City), who sponsored the House bill on nitrogen hypoxia, told The Huffington Post via email that "nitrogen hypoxia is a painless form of capital punishment that is simple to administer, doesnt depend upon the aid of the medical community, and is not subject to the supply constraints we are faced with when using the current three drug cocktail protocol." (Supply for the three-drug lethal injection cocktail was disrupted after its European manufacturer refused to further supply the drug to the U.S. for executions.)

Christian noted the idea for nitrogen hypoxia came from a 2014 Slate article on the subject. No countries in the world use nitrogen gas as a state-sanctioned execution method, according to the article.

Oklahoma state Sen. Anthony Sykes (R-Moore), who sponsored the state Senate version of the nitrogen hypoxia bill, told the Associated Press the method is "recognized as the most humane by those who oppose the death penalty, adding that "it causes a very quick and sudden loss of consciousness and of life almost simultaneously." Sykes did not cite a specific expert or entity in his claim and did not immediately respond to The Huffington Posts request for comment.

But Fordham Law Professor Deborah Denno, one of the nation's foremost death penalty experts, said such claims are similar to ones death penalty supporters made about lethal injection in the 1970s.

If you look at all the statements and newspaper clippings made in 1977 when lethal injection was introduced [in Oklahoma], they sound very similar, Denno told The Huffington Post. You would read comments about how this would be painless and immediate."

Dr. Joel Zivot , assistant professor of anesthesiology and surgery at Emory University School of Medicine, told HuffPost it's ethically impossible for a doctor to conduct tests -- and therefore reach conclusions -- on execution procedures.

"No physician is an expert in killing, and medicine doesnt position itself intentionally in taking a life," Zivot said. He added, "Theres no therapeutic use of nitrogen gas, and theres no way to ethically or practically test if nitrogen gas is a humane alternative."

Meanwhile, Utah is considering a measure to bring back firing squads if its unable to maintain its supply of lethal injection drugs. In May 2014, Tennessee lawmakers authorized a re-use of the electric chair as a back-up to lethal injection. Months later, Tennessee inmates sued the state and called the chair an unconstitutional "torture device.

Lethal injection rose to prominence in the early 1990s and is the primary method of execution in the 32 states that still allow the death penalty. Other methods may still be used, typically at the inmates discretion. Eight states still have the electric chair, four have the gas chamber, three still permit hanging and two allow firing squads on certain technicalities. The last use of the gas chamber was in Arizona in 1999.

Both experts and capital punishment abolitionists have criticized the secretive nature of many state executions. States are less than forthcoming about many details of the procedure, including protocols; the identity of drug manufacturers; the identity of prison personnel involved in executions; and what personnel training for executions entails. (Medical professionals are ethically barred from participating in executions and are only present to declare time of death.) In 2014, The Guardian, The Associated Press and three Missouri newspapers sued Missouri for withholding such information. Similar lawsuits were filed by Ohio death row inmates last year.

Denno said since execution methods don't have trial runs, any new or adjusted protocol is effectively an experiment on the inmate.

"You cant ask a person who was executed if their death was cruel," Zivot said.

Denno added that what little research is available has suggested that the gas chamber is the most painful form of punishment. "Theres been a bit of a consensus that lethal [cyanide] gas has been the most egregious [method], she said. Theres no question that people are dying a slow death in a very painful way.

While gas chamber victims slowly suffocate, Denno said, electrocution imparts an extra indignity by leaving its victims mutilated.

"Some people scream out when the electricity is first being applied, but you're essentially burning to death, Denno explained. "Your body fluids are boiling. Ones eyeballs can pop out -- thats why they put a cap over peoples head."

In other instances, like that of the 1997 Florida execution of Pedro Medina, the head, skin or hair can catch on fire mid-execution.

Ironically, Denno said, firing squads are perhaps the most effective execution method. "Weve had three firing squad executions in the modern area -- since the '70s -- that have gone off without a hitch, she said.

Zivot criticized Oklahoma as having shown "a lack of seriousness" about determining whether its methods meet both ethical and constitutional requirements.

Youre left with the state declaring this to be safe and a form of execution thats not needlessly cruel," Zivot said. "I would ask the state, Prove that.