Executioners gave a convicted killer in Arizona two full doses of a lethal drug cocktail injection before he succumbed to death almost two hours after the execution commenced, according to the state's assistant attorney general.

Joseph Wood, 55, a convicted double-murderer, gasped for breath for over 90 minutes on Wednesday after his execution began, according to his attorneys. The Arizona governor vowed to launch an internal probe into the "botched" execution but said justice was served through the process, Reuters relayed.

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State corrections officials denied the execution was botched and maintained that Wood did not feel any pain as he was fully sedated the whole time. In a statement, Department of Corrections director Charles Ryan said the IV team including a licensed physician verified several times that Wood was not in pain and that he was in coma.

The prolonged process of Wood's execution came on the heels of two separate cases in Ohio and Oklahoma this year where the lethal injections failed. It also revived debates over death penalty in the U.S. and led to the suspension of other executions amidst an internal investigation.

According to an Arizona Republic reporter who watched the execution, Wood gasped 660 times within 90 minutes before he fell silent. Assistant Attorney General Jeffrey Zick said Wood received two full doses of the lethal drug combination when the procedure was conducted.

Zick told a judge during an emergency hearing in the middle of the execution that the condemned inmate was "brain dead" and he was just having "involuntary reactions" similar to what a person would manifest after life support is removed.

Wood was convicted in 1991, two years after he fatally shot his former girlfriend Debbie Dietz and her father, Gene Dietz. He was the fifth individual to undergo execution in Arizona in the last two years.

Defense lawyers said the incident was the "most prolonged bungled execution" and demanded an independent probe into the process. Attorney Dale Baich said the people of Arizona need to know what drugs were used and how much was given to Wood to provide transparency in the process which he described as "cloaked in secrecy."

In January, an execution of an Ohio convict gasped for breath for 25 minutes after he was injected with a lethal combination of midazolam and hydromorphone before he died. In April, an Oklahoma death row inmate's needle was dislodged mid-execution and the process was stopped, but he had a fatal heart attack.

Ohio was the first U.S. state to use the midazolam-hydromorphone combination in a lethal injection. Arizona previously said it would use the same lethal drug cocktail but would administer it at a higher dose.

Tagsjoseph wood, arizona botched execution, death by lethal injection