Joseph Wood is seen in this booking photograph from the Arizona Department of Corrections. Picture: Arizona Department of Corrections, via Reuters

Phoenix - Lawyers for a convicted double-murderer whose lethal injection in Arizona dragged on for two hours, while witnesses watched him gasping for breath and attorneys scrambled to halt the process, have called for an outside review of the “horrifically botched execution.”

The ordeal in putting Joseph Wood to death on Wednesday at a prison facility south-east of Phoenix marked the third instance this year of a lethal injection gone awry, after mishaps in Ohio and Oklahoma that renewed the US debate over capital punishment.

“He gasped and struggled to breathe for about an hour and 40 minutes,” said Dale Baich, one of Wood's lawyers, who watched the execution and tried in vain to stop it. He called for an independent inquiry.

An Arizona Republic journalist who witnessed the event said he counted Wood gasping for air about 660 times before the 55-year-old inmate fell silent.

During that time, defence attorneys took the extraordinary step of filing emergency court petitions seeking to cut short the procedure and resuscitate their client, arguing Wood was being subjected to unconstitutionally cruel and unusual punishment.

But US Supreme Court Justice Anthony Kennedy denied the appeal, and Wood was pronounced dead at 3.49pm local time, one hour and 57 minutes after the execution had officially begun.

State Corrections Director Charles Ryan disputed suggestions that Wood had suffered, saying in a statement that once sedated - five minutes into the procedure - the inmate “did not grimace or make any further movement.”

Ryan characterised Wood's breathing as “sonorous respiration, or snoring,” and said execution team members with whom he conferred during the process assured him “unequivocally that the inmate was comatose and never in pain or distress.”

He added that the time it takes to complete an execution varies for each individual.

Arizona Governor Jan Brewer expressed concern over how long the procedure lasted and ordered a review by prison officials.

But Baich insisted the inquiry should be independent, saying that in opposing his client's earlier appeals, the state had “fought tooth and nail to protect the extreme secrecy surrounding its lethal injection drugs and execution personnel.”

“An independent investigation, led by someone outside of the Department of Corrections and outside of the executive branch of state government, must fully explore the practices which led to tonight's horrifically botched execution,” he said.

Brewer for her part insisted in a statement that justice was done, that Wood had “died in a lawful manner, and by eyewitness and medical accounts he did not suffer.”

“This is in stark comparison to the gruesome, vicious suffering that he inflicted on his two victims, and the lifetime of suffering he has caused their family.”

Wood was found guilty in 1991 of fatally shooting his former girlfriend, Debbie Dietz, 29, and her father, Gene Dietz, 55, two years earlier at a family automobile body shop in Tucson.

Arizona's Supreme Court cleared the way for Wood to be put to death on Wednesday, lifting an 11th-hour stay of execution that had briefly been granted on the basis of questions he raised about the mix of drugs to be administered to him.

In January, convicted killer Dennis McGuire was put to death in Ohio using a sedative-painkiller mix of midazolam and hydromorphone, the first such combination used for a US lethal injection. The execution took about 25 minutes, with McGuire reportedly convulsing and gasping for breath.

Arizona had said it would use the same combination of drugs on Wood, but at higher doses.

A different sort of mishap occurred in April in Oklahoma, where killer Clayton Lockett writhed in pain as a needle became dislodged during his lethal injection. The process was halted in that case, but Lockett died shortly after of a heart attack. - Reuters