SAN FRANCISCO - A third execution by lethal injection has gone awry in six months, renewing debate over whether there is a foolproof way for the government to humanely kill condemned criminals, and whether it's even worth looking for one.

Death penalty opponents say any killing is an unnecessarily cruel punishment. Proponents may favor the most humane execution method possible, but many reject the idea that a few minutes or hours of suffering by a criminal who caused great suffering to others should send government back to the drawing board.

Thirty years ago, states and the federal government gave little thought to the condemned inmates' comfort. Most executioners used electric chairs, but death row inmates were also hanged, put to death in the gas chamber or faced a firing squad.

Mistakes occurred. Inmates appeared to suffer in the gas chamber. Electric chairs caught fire or malfunctioned and didn't kill. So a growing number of law enforcement officials, legislators and advocates began searching for a foolproof, constitutional method for executions.

In 1977, an Oklahoma medical director appeared to have found a solution. Dr. Jay Chapman came up with a three-drug combination that promised to put the inmate to sleep before painlessly and quickly drifting off to death. Chapman's formula replaced the state's use of the electric chair.

Now, calls are mounting to scrap lethal injection, even by those who support capital punishment like Chief Judge Alex Kozinski of the 9th U.S. Circuit Court of Appeals. He believes a completely humane method of execution isn't possible and favors firing squads.

"If we as a society cannot stomach the splatter from an execution carried out by a firing squad, then we shouldn't be carrying out executions at all," Kozinski wrote Monday in support of carrying out Joseph Rudolph Wood's execution.

Wood gasped for more than an hour and a half during his execution Wednesday before he died.

Chapman's three-drug combination became the default execution method for the federal government and in every state - some three dozen - that has capital punishment. Lethal injection was embraced as the best possible way to execute and the apparent painless and swift death it caused were seen as attributes to counter lawsuits and protests that claimed capital punishment was cruel and unusual.

Since then, more than 1,200 inmates have been executed by lethal injection. The U.S. Supreme Court in 2008 ruled the method constitutional.

"Execution by lethal injection should be a humane way to die," said Arthur Caplan, head of medical ethics at New York University's Langone Medical Center. "But it isn't."

Caplan said that there hasn't been any reported trouble with physicians who in some states can legally help people commit suicide. "So we know it can be done painlessly," he said.

But medical ethicists and professional licensing boards for doctors and nurses forbid their participation in executions, which are carried out by lay workers who sometimes struggle with administering a lethal injection.

Further, pharmaceutical companies are refusing to ship prisons the three drugs necessary to mimic Chapman's mixture. That has caused prison officials to scramble to find alternative drugs that may not kill as quickly. Anesthesiology experts say they're not surprised that the two drugs Arizona used Wednesday took so long to kill Wood.

So the lawsuits and protests persist. So do the problems.

On Wednesday, Wood gasped for air for 90 minutes and took about two hours to die after receiving an injection. An Ohio inmate gasped in similar fashion for nearly 30 minutes in January. An Oklahoma inmate died of a heart attack in April, minutes after prison officials halted his execution because the drugs weren't being administered properly.

Previously, non-medical personnel had trouble delivering the lethal injection or had trouble finding veins on longtime drug abusers. When doctors were called in to assist, the American Medical Association objected that it was unethical for physicians to be directly involved in executions.

After questions over the amount of time it took for Wood to die, Arizona Gov. Jan Brewer ordered a review of the state's execution protocol. Governors in Ohio and Oklahoma have ordered similar reviews.

Erwin Chemerinsky, dean of the University of California, Irvine law school and death penalty opponent, said he believes lethal injection is here to stay because the U.S. Supreme Court ruled it constitutional in 2008 - and because the public would never stand for firing squads.

"It is far too gruesome," Chemerinsky said.

However, a Utah lawmaker says he will introduce legislation next year to restart firing squads in that state. Utah eliminated firing squads in 2004, citing the excessive media attention it gave inmates. But those sentenced to death before 2004 can choose it, which is what condemned inmate Ronnie Lee Gardner did in 2010. Five police officers using .30-caliber Winchester rifles executed Gardner, the third inmate to die that way since the U.S. Supreme Court reinstated the death penalty in 1976.

The idea of using firing squads has come up in Wyoming, where an interim legislative committee has been looking at the issue this summer. A proposed bill to allow firing squads failed an introductory vote last winter in the state Legislature.

Kent Scheidegger, legal director of the pro-death penalty Criminal Justice Legal Foundation in Sacramento, said that the lethal injection process can be fixed and he wants government to looking for an answer.

Regardless, he said it's unclear whether Wood - and others who endured prolonged lethal injections- suffered any pain, which is more than his two victims could say.

California Attorney General Kamala Harris, a death penalty opponent who must decide whether to appeal a federal judge's recent decision striking down California's death penalty, was asked this week if a humane execution method is even possible.

"I honestly don't know," she said.