For the third time this year, a state has set out to execute a convicted murderer but found itself unable to cleanly carry out the sentence. The continuing problems have raised questions about whether the nation’s current methods for carrying out capital punishment — a patchwork of drugs, dosages and needle techniques — can result in a humane death.

Here is a primer to help understand the controversial execution in Arizona on Wednesday and how it has again put the question of capital punishment in the United States under a spotlight.

Question: What happened in Arizona?

Answer: After exhausting all of his appeals in the federal and state judicial systems, Joseph Rudolph Wood III 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, in what was one of the longest executions in U.S. history. Convicted of a double murder, Wood, 55, gasped 660 times, according to Michael Kiefer, a reporter with the Arizona Republic, who witnessed the execution.

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

A: It isn’t the length of time that is the issue, but whether his punishment was cruel and unusual, which would be prohibited by the U.S. Constitution. The U.S. Supreme Court has upheld society’s right to execute prisoners, but has insisted that all of the proper constitutional guarantees be met.

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, including relatives of those slain by Wood, argue that he died in his sleep and that the gasps were no more significant than the snoring sounds that often precede death. One relative said it didn’t matter whether Wood was in pain, given the horrible nature of his crimes.

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

A: No, there have been problems in Ohio and in Oklahoma.

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. Ohio officials have defended their actions but stayed further executions pending a review. They have said they will increase the dosages of lethal-injection drugs.

In April, Clayton Lockett writhed and grimaced during what probably was the most obvious of the controversial executions, this one in Oklahoma. According to witnesses and official comment from the state, 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 about 10 minutes later a doctor announced that Lockett was unconscious. The execution team then began administering the next drugs in the protocol, one to paralyze him and one to make his heart stop.

Lockett didn’t die as expected. Instead, he began to wiggle, clenched his teeth and even tried to raise his head. Witnesses reported he was able to speak. Prison officials pulled a curtain to block any further view of the activities in the chamber.

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 and pledged further investigation.

Q: Have there always been problems in executing inmates?

A: There have always been questions about techniques, such as hanging, electrocution and the use of firing squads. All are lethal, but are they humane and do they meet the constitutional requirement to avoid cruel and unusual punishment?

The use of a lethal injection, containing an anesthetic and a paralyzing agent, became popular because this method has been widely considered more humane than the old-fashioned ways, until recently, at least.

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. That touched off problems for many states, forcing them to seek other sources for drugs needed for executions.

Q: Where do the states get the drugs now?

A: Finding an adequate supply has been a problem, so states have been experimenting with different drugs, combinations and dosages. Exactly from where the drugs come 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.

Attorneys seeking to block executions have fought these secrecy laws, arguing that the information is needed to determine whether capital convicts would face cruel and unusual punishment. States in general have defended the secrecy laws, citing security concerns and a desire to protect suppliers of the drugs.

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. Several states turned to midazolam after they ran out of the more powerful barbiturate drugs. In Oklahoma, the state used a three-drug cocktail of midazolam, vecuronium bromide and potassium chloride, a combination never before used in that state.

Q: What’s next?

A: States probably will continue to try different dosages and drugs as they seek a more reliable protocol. Attorneys seeking to block executions will continue to argue that inmates should be told the identity of suppliers and given other needed information, while states probably will seek to continue to protect their sources. Federal or state courts — Georgia, Louisiana, Missouri, Oklahoma and Texas — have permitted executions to take place despite such challenges to secrecy.

So far, the U.S. Supreme Court has refused to intervene.