Please explain ... Oklahoma Governor Mary Fallin has delayed all executions in the state as an investigation gets underway. Picture: AP

Governor Mary Fallin said in a statement on Thursday that “it became apparent” during discussions with prison officials last week that the Department of Corrections used potassium acetate — not potassium chloride, as required under the state’s protocol — to execute 47-year-old Charles Frederick Warner in January.

Warner was put to death for killing his roommate’s 11-month-old daughter in 1997.

After the drug was administered he twitched uncontrollably in the death chamber and said, “My body is on fire”.

“Until we have complete confidence in the system, we will delay any further executions,” Fallin said.

It’s the same drug that led to the postponement of another inmate’s death, Richard Glossip, last week. That delay came after the state’s department received a shipment of potassium acetate instead of potassium chloride.

Charles Frederick Warner ... the convicted baby killer said ‘My body is on fire’ as he died. Picture: AP Photo/Oklahoma Department of CorrectionsSource:AP

Citing Warner’s autopsy report, The Oklahoman reported on Thursday that the Office of the Chief Medical Examiner received two syringes labelled “potassium chloride,” but that the 12 vials used to fill the syringes were labelled “single dose Potassium Acetate Injection.”

That contradicts the official log of Warner’s January 15 execution, initialled by a prison staffer, which indicated that the state properly used potassium chloride to stop his heart, according to a copy of the log obtained by The Associated Press.

“We cannot trust Oklahoma to get it right or tell the truth,” said Dale Baich, a lawyer representing Oklahoma death row inmates who are challenging the state’s lethal injection protocols.

“We will explore this in detail through the discovery process in the federal litigation.”

Fallin declined to say if she still has confidence in prisons director Robert Patton. She said she would wait until Attorney General Scott Pruitt completes an investigation into both Warner’s execution and last week’s mix-up.

“I want to let the attorney general do his job first, tell us what’s factual and what’s not, give us the information, and then we’ll make a judgment then,” Fallin said.

Patton oversaw both Warner’s execution and the April 2014 lethal injection of Clayton Lockett, who writhed on the gurney, moaned and pulled up from his restraints.

Execution team members considered trying to save his life and even taking Lockett to an emergency room before he finally died, 43 minutes after his initial injection.

On death row ... inmate Richard Glossip. Picture: AP Photo/Oklahoma Department of CorrectionsSource:AP

Warner had been scheduled to be put to death the same night, until Lockett’s execution went awry.

Potassium chloride, which stops the heart, is the final drug in the state’s three-drug protocol, following the application of a sedative, midazolam; and a paralytic, rocuronium bromide, which prevents normal breathing.

Patton said last week that prison authorities discovered last week’s error and immediately contacted the supplier, “whose professional opinion was that potassium acetate is medically interchangeable with potassium chloride at the same quantity.”

But Dr Alice Chen, an internal medicine specialist and executive director of Doctors for America, told the AP that the two drugs are not interchangeable.

“We’re not certain what the dose should be, how different people would react to it in the cocktail,” she said.

Fallin said she was not told that the wrong drug may have been used to execute Warner until last week.

“I was not aware, nor was anyone in my office aware, of that possibility until the day of Richard Glossip’s scheduled execution,” she said. “

“It is imperative that the attorney general obtain the information he needs to make sure justice is served competently and fairly.”

Last week, the Death Penalty Information Center said potassium acetate had never been used in a US execution.