The two-hour-long execution of a US inmate last month required 15 doses of a relatively untested drug cocktail, according to documents, prompting renewed calls for an independent investigation.
Arizona's July 23 execution by lethal injection of double murderer Joseph Wood was drawn out for an agonizing 117 minutes, during which the condemned man gasped and snorted, rather than the the usual 10.
His attorney late Friday released 331 pages of documents he had received from the Arizona Department of Corrections, which examined Wood after the procedure, and said the use of that quantity of medication was a violation of official procedures.
"The Arizona execution protocol explicitly states that a prisoner will be executed using 50 milligrams of hydromorphone and 50 milligrams of midazolam," lawyer Dale Baich said.
"The execution logs released today by the Arizona Department of Corrections shows that the experimental drug protocol did not work as promised. Instead of the one dose as required under the protocol, ADC injected 15 separate doses of the drug combination, resulting in the most prolonged execution in recent memory," he added.
"This is why an independent investigation by a non-governmental authority is necessary."
Wood's execution was the third this year to have gone on far longer than usual, as US states experiment with an untested combination of lethal medications amid a shortage of tried and tested drugs after European manufacturers halted exports.
The drug cocktail used to kill Wood -- a combination of the sedative midazolam and the painkiller hydromorphone -- had only been used once before -- in January in Ohio, where it took inmate Dennis McGuire 26 minutes to die.
Death penalty opponents allege that the lengthy execution of Wood, 55, and others amount to a form of torture or the "cruel and unusual" punishment forbidden by the US Constitution.
US courts have rejected several appeals by inmates concerned by the procedure, amid predictions that the debate will likely eventually have to be decided by the US Supreme Court.
Eighteen US states have abolished the death penalty, but others -- and the federal government -- maintain the practice, and polls suggest it retains majority support among the US public.