(CNN) -- Arizona spent two hours killing death row inmate Joseph Wood this week, an unusually long time for an execution. Wood's death has reopened the debate about capital punishment and lethal injection. Lethal injection is used in all 32 states that have the death penalty.
Some witnesses said Wood was gasping for breath and seemed to be in pain. Others said he was simply snoring. The state of Arizona said Wood didn't suffer. Either way, there are renewed concerns that executions, which are supposed to be quick and painless, are neither.
Typically executions are performed using three drugs in stages. The first drug is an anesthetic. The second drug is called a paralytic and the third drug is supposed to stop the heart.
For years, executioners used a drug called sodium thiopental as the first drug, the anesthetic, until the only U.S. producer of the drug stopped making it. Then the United States turned to European manufacturers, but they refused to sell the drug for use in executions.
Since sodium thiopental was taken off the market for executions, states have turned to a drug called midazolam for the anesthetic. But experts believe there have been problems with the drug in at least three executions. The problems described by witnesses included gasping, snorting or choking sounds and, in one execution, the inmate was described as speaking.
Wood was injected with midazolam on Wednesday, as well as hydromorphone, a narcotic painkiller that, with an overdose, halts breathing and stops the heart from beating. It is one of the new combinations that states have tried, with some controversial results.
What would qualify as a botched execution? Michael Radelet, a professor of sociology and law, said an execution is botched when it looks like the inmate endured "prolonged suffering" for 20 minutes or more.
Whether they're "botched" or not, plenty of executions don't go by the book. Most of the executions listed here were compiled by Human Rights Watch.
Dennis McGuire, executed January 16, 2014, in Ohio. Columbus Dispatch reporter Alan Johnson said the execution process took 24 minutes, and that McGuire appeared to be gasping for air for 10 to 13 minutes. He had been injected with midazolam and hydromorphone, a two-drug combination that hadn't been used before in the United States. McGuire, 53, was convicted in 1994 in the rape and murder of a 22-year-old pregnant woman.
Clayton Lockett, executed April 29, 2014, in Oklahoma. Lockett was injected with midazolam, but instead of becoming unconscious, he twitched, convulsed and spoke. The execution was halted, but Lockett died after 43 minutes. A team that prepared Lockett for execution failed to set a properly functioning IV in his leg, according to preliminary findings of an independent autopsy. Lockett was convicted in the 1999 death of an Oklahoma woman who was buried alive after she was raped and shot.
Jose High, executed November 7, 2001, in Georgia. This execution illustrates a common problem. The execution team had trouble finding a usable vein and spent 39 minutes looking before finally sticking a needle into High's hand. A second needle was inserted between his neck and shoulder by a physician. Sixty-nine minutes after the execution began, he was pronounced dead. High was convicted in a 1976 store robbery and the kidnapping and murder of an 11-year-old boy.
Claude Jones, executed December 7, 2000, in Texas. The execution team spent 30 minutes looking for a suitable vein, a difficult task because of Jones' history of drug abuse. He was convicted in the 1990 murder of a liquor store owner.
Joseph Cannon, executed April 23, 1998, in Texas. The needle popped out and Cannon said to witnesses, "It's come undone." The needle was reinserted and 15 minutes later a weeping Cannon made his second final statement. He was convicted of murder.
John Wayne Gacy, executed May 10, 1994, in Illinois. Lethal chemicals solidified and clogged in the IV tube leading to Gacy's arm. A new tube was installed and the execution proceeded. Gacy, one of America's most notorious killers, was convicted in 1980 of raping and killing 33 boys and young men he lured into his home.
Charles Walker, executed September 12, 1990, in Illinois. The execution was prolonged because a kink in the plastic tubing stopped the flow of chemicals into Walker's body and an intravenous needle pointed at Walker's fingers, instead of his heart, said a Missouri State Prison engineer hired to assist in the execution. Walker was convicted of two counts of murder.
Raymond Landry, executed December 13, 1988, in Texas. The catheter dislodged and flew through the air two minutes after injection of drugs into Landry's body. The execution team spent 14 minutes inserting it again and Landry was pronounced dead 40 minutes after being strapped to the gurney. He had been convicted of murder.
Shelby Lin Erdman contributed to this report