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An Arizona execution took nearly two hours on Wednesday, and witnesses said the inmate gasped and snorted for well over an hour after the lethal injection.

The execution of double-murderer Joseph Wood — which Arizona carried out with a two-drug combination it had never before tried — is certain to fan the debate over how U.S. states carry out the death penalty.

"I've never witnessed an execution that took that long," Wood's federal public defender, Dale Baich, told NBC News. "The state of Arizona today conducted a failed experiment. ... It was horrible to watch."

Media witnesses said the execution — which followed a botched lethal-injection in Oklahoma, and a protracted one in Ohio using the same drugs — began as expected with doctors inserting the IVs into Wood's veins.

The witnesses said Wood, 55, thanked Jesus as his savior and delivered these final words:

"I take comfort knowing today my pain stops, and I said a prayer that on this or any other day you may find peace in all of your hearts, and may God forgive you all."

"At a certain point you wondered if he ever was going to die"

Then, as the dose was administered, Wood closed his eyes and appeared sedated. But after a few minutes, witnesses said, they could see something was wrong.

“It was almost like snoring," Associated Press reporter Astrid Galvan said. "It looked like he was yawning almost.”

Wood would go on to gasp more than 600 times over the course of an hour and 40 minutes, witnesses said. One witness likened it to the movements a fish makes when it's taken out of water.

"At a certain point you wondered if he ever was going to die," reporter Troy Hayden said.

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The Department of Corrections said the medical team checked on Wood eight times and declared that he was "deeply sedated" or "comatose" and "never in pain or distress."

Midway through the execution, Baich filed papers asking a federal court to stop the execution and order prison officials to try to resuscitate Wood. But before the court could act, Wood was pronounced dead.

"The execution commenced at 1:52 p.m. at the Arizona State Prison Complex (ASPC) — Florence. He was pronounced dead at 3:49 p.m," a statement from Arizona Attorney General Tom Horne said.

Baich said the execution clearly violated the Constitution's ban on cruel or unusual punishment and was an ordeal that "could have been prevented."

"The state of Arizona was on notice," he said. "They knew that Ohio had used these two drugs back in January and they had problems with that execution. Why they used the same two drugs for Mr. Wood's execution I don't know."

Baich said he had asked a judge to order that Wood's tissue and blood samples, along with labels from the drugs that were used, be preserved for further investigation.

Wood was condemned to die for the 1989 murder of his 29-year-old girlfriend, Debra Dietz, and her father. The victims' families said the focus should be on his crimes, not his death.

"I don't believe he was gasping for air. I don't believe he was suffering," said Dietz's sister, Jeannie Brown. "Who really suffered was my dad and my sister when they were killed."

Speaking to The Associated Press, Dietz's brother-in-law Richard Brown added: "This man conducted a horrifying murder and you guys are going, 'let's worry about the drugs. Why didn't they give him a bullet? Why didn't we give him Drano?"

Gov. Jan Brewer said she was "concerned" about the execution length and ordered a review but insisted Wood did not suffer "in stark comparison to the gruesome, vicious suffering that he inflicted on his two victims."

The ACLU called for a moratorium on executions until states can ensure that their lethal injections will work as intended.

“It’s time for Arizona and the other states still using lethal injection to admit that this experiment with unreliable drugs is a failure," the group said in a statement. "Instead of hiding lethal injection under layers of foolish secrecy, these states need to show us where the drugs are come from. Until they can give assurances that the drugs will work as intended, they must stop future executions.”

Wood had challenged the execution on the grounds that the state was violating the First Amendment by keeping the source of the lethal-injection drugs secret.

"This is something that should be be part of the public debate and not dealt with in secret," Baich said Wednesday.

An appeals panel agreed with him, but the U.S. Supreme Court lifted the stay of execution. The Arizona Supreme Court briefly delayed the execution on Wednesday morning, but ultimately gave the state the green light.

Wood was put to death with a combination of midazolam and hydromorphone, the same drugs used in Ohio's execution of Dennis McGuire, who seemed to struggle for air and took 25 minutes to die.

States have been trying new injection combinations and getting them from controversial compounding pharmacies — often under a shroud of secrecy — because manufacturers refuse to sell them to prison for executions.

Wood's execution date had been put on hold several times as the case wound its way through last-minute appeals. One of those decisions was notable for a dissent in which the chief judge of a federal appeals court said the guillotine would be better than lethal injection for executions.

Deborah Denno, professor of criminal law and criminal procedure at Fordham Law School, told The Associated Press it may be up to Legislatures or the public to bring any change. "I think every time one of these botches happens, it leads to questioning the death penalty even more," she said. "It will reach a point where the public will question the value of these execution procedures generally, and perhaps the death penalty itself."

NBC News' M. Alex Johnson and The Associated Press contributed to this report.