PHOENIX — Lawyers for an inmate who was executed last month by lethal injection said Friday that his executioners injected him with 15 times the standard dose of a sedative and a painkiller during a procedure that lasted nearly two hours before their client was declared dead.

The execution of the inmate, Joseph R. Wood III, which was the fourth troubled one in the nation this year, renewed debate over the death penalty and prompted Arizona’s attorney general to order a temporary halt to executions in the state. The Arizona Department of Corrections announced Friday that it was seeking an outside investigator to conduct an independent inquiry into Mr. Wood’s execution.

Mr. Wood was executed on July 23 for the murders of his girlfriend and her father in 1989. He was injected with a two-drug combination of hydromorphone, an opioid painkiller that suppresses breathing, and midazolam, a sedative.

“The Arizona execution protocol explicitly states that a prisoner will be executed using 50 milligrams of hydromorphone and 50 milligrams of midazolam,” Dale A. Baich, one of the lawyers who represented Mr. Wood, said in a statement. “The execution logs released today by the Arizona Department of Corrections show that the experimental drug protocol did not work as promised.”