An autopsy report showed Oklahoma prison officials used an unapproved drug in the January execution of convicted murderer, whose final words were "my body is on fire."

The third drug applied in the lethal injection was potassium acetate, when it should have been a different chemical, potassium chloride, The Oklahoman reported. The newspaper called the step a violation of protocol on the part of Oklahoma Corrections Department officials.

The execution of Charles Warner, who had been convicted of the rape and murder of an 11-month-old in 1997, made national headlines after witnesses in the death chamber relayed his final comments, Reuters noted. Warner's lawyers had previously been unable to halt the execution, even though they had said that the drug mix could cause undue pain and suffering.

Dale Baich, one of his attorneys, said Warner's newly released autopsy report "yet again raises serious questions about the ability of the Oklahoma Department of Corrections" to properly carry out executions, according to the Wall Street Journal.

The mistaken drug, meanwhile, was almost employed for a second time on Sept. 30, but Oklahoma Gov. Mary Fallin granted a last-minute stay of the scheduled execution of another convicted murderer, Richard Glossip. The governor's decision came after she learned of the mix-up in Warner's lethal injection, the Oklahoman detailed.

The state's attorney general, Scott Pruitt, has since launched a probe into the circumstances surrounding Glossip's execution. The inquiry will also look into any previous drug mistakes, he said on Wednesday.

"I want to assure the public that our investigation will be full, fair and complete and includes not only actions on Sept. 30, but any and all actions prior, relevant to the use of potassium acetate and potassium chloride," Pruitt said.

The attorney general also asked the Oklahoma Court of Criminal Appeals to postpone three coming executions indefinitely, a request the court granted on Friday, the Wall Street Journal detailed.

Mother Jones, meanwhile, recalled that it was an Oklahoma coroner, Jay Chapman, who wrote the modern lethal injection protocol in an effort to make executions more humane. But "it never occurred to me when we set this up that we'd have complete idiots administering the drugs," Chapman told The New York Times in 2007.