For the third time this year, a state was unable to cleanly carry out a capital punishment sentence, raising questions about the nation's current methods. A primer:

Q: What happened in Arizona?

A: Joseph Rudolph Wood was taken to Arizona's death chamber on Wednesday, and a lethal injection was administered at 1:57 p.m. Wood was pronounced dead at 3:49 p.m., nearly two hours later. Convicted of a double murder, Wood, 55, gasped 660 times, according to Michael Kiefer, an Arizona Republic reporter who witnessed the execution.

Q: Why does it matter how long the execution takes?

A: It isn't the length of time that matters, but whether his punishment was cruel and unusual, which is prohibited by the U.S. Constitution.

Q: Was this execution botched?

A: Opponents of the death penalty say yes, arguing that Wood appeared to be in pain and was tortured during his execution. Proponents of the death penalty argue that he died in his sleep and that the gasps were no more significant than the snoring sounds that often precede death.

Q: Is this the first time there has been a problem with the lethal injection protocol?

A: In January, convicted killer Dennis McGuire made snorting and gasping sounds as he was executed in Ohio by lethal injection. His family has sued and sought a moratorium on all executions in the state. In April, Calvin Lockett writhed and grimaced during his execution in Oklahoma. Lockett, a convicted killer and rapist, was taken to the execution chamber and an intravenous line was inserted into his vein. A sedative was administered, and a doctor announced that Lockett was unconscious. The execution team began administering the next drugs, one to paralyze him and one to make his heart stop. Instead of dying, Lockett clenched his teeth and tried to raise his head. Prison officials pulled a curtain. They later said Lockett died of a heart attack after 43 minutes. They said the problems were caused by a failure of a vein that allowed the drugs to seep into Lockett's tissue.

Q: Why did the states change their original drug protocols?

A: Until about 2009, most states used a three-drug combination: a powerful anesthetic such as pentobarbital, a paralyzing agent such as pancuronium bromide and something to stop the heart, often potassium chloride. But by 2010, many suppliers, particularly companies in Europe, where the death penalty is generally banned, came under public pressure and stopped making their medications available for executions.

Q: Where do the states get the drugs now?

A: States have been experimenting with different drugs, combinations and dosages. Exactly from where the drugs come from is generally not known because most states have laws that protect the identities of the suppliers and even the names and dosages of the drugs used.

Q: What was used in Arizona?

A: Arizona used a two-drug combination of midazolam, a sedative and muscle relaxant, and hydromorphone, a powerful member of the opioid class. It is the same drug combination that caused problems in Ohio.

Los Angeles Times