Oklahoma used the wrong drug to execute Charles Warner in January, according to autopsy records obtained by the The Oklahoman on Thursday.

The state uses a three-drug cocktail to execute death-row inmates. First, the sedative midazolam is injected, followed by pancuronium bromide, a paralytic. The third drug, potassium chloride, is then used to stop the heart. But according to the autopsy records, Oklahoma used vials of potassium acetate instead of potassium chloride in Warner’s lethal injection. The state’s execution protocol only allows the use of potassium chloride.

The drug mix-up first came to light when Oklahoma Governor Mary Fallin issued a last-minute stay of execution for Richard Glossip on September 30 after corrections officials discovered they had purchased potassium acetate instead of potassium chloride. On October 2, the Oklahoma Court of Criminal Appeals imposed an indefinite stay on all executions at the request of the state attorney general as investigations proceed.

This is the first known execution in the U.S. to use a wrong lethal-injection drug, according to Robert Dunham, the executive director of the Death Penalty Information Center. “But before today, we had never heard of a state using potassium acetate in an execution, either,” he added. “This is one of the problems with the secrecy procedures that many states now have in place.”

The latest revelation adds further turmoil to Oklahoma’s troubled capital-punishment system. Warner, who received a death sentence for the rape and murder of a child, was one of three death-row inmates who sued Oklahoma to halt their executions after the botched execution of Clayton Lockett in 2014. After Lockett spent almost an hour writhing and gasping before his death, Warner and the inmates argued that midazolam’s use in their own lethal injections would violate the Eighth Amendment. The U.S. Supreme Court denied Warner’s request for a stay on January 15, with four justices issuing a rare written dissent.

The inmates’ case focused on midazolam, but the justices also recognized the changing landscape of U.S. execution methods. “The questions before us are especially important now, given States’ increasing reliance on new and scientifically untested methods of execution,” wrote Justice Sonia Sotomayor in her dissent from the denial of Warner’s stay request.

One week later, the Court granted the remaining inmates’ petition to hear the case and renamed it Glossip v. Gross after one of the surviving inmates. In July, the Supreme Court upheld Oklahoma’s lethal-injection protocol in a deeply divided 5-4 decision. In the majority opinion, Justice Samuel Alito wrote that after Lockett’s botched execution, Oklahoma “executed Warner using these revised procedures and the combination of midazolam, a paralytic agent, and potassium chloride.”

The Oklahoma governor’s office told The Oklahoman that potassium chloride and potassium acetate are medically indistinguishable for their purposes. Capital defense attorneys argue that the mix-up highlights the secrecy problems surrounding the state’s execution procedures. “We cannot trust Oklahoma to get it right or to tell the truth,” said Dale Baich, a federal public defender who helped represent the death-row inmates in Glossip, in a statement. “The execution logs for Charles Warner say that he was administered potassium chloride, but now the State says potassium acetate was used.”

Potassium acetate can be normally used to treat potassium deficiencies, according to the U.S. Food and Drug Administration. Another common use for the chemical is deicing airport runways. In 2009, a U.S. Geological Survey study estimated that two-thirds of U.S. airports that deice their runways used potassium acetate to do so. The USGS study also warned that potassium acetate runoff may cause environmental damage and harm nearby aquatic life.