The Oklahoma State Penitentiary. (Nick Oxford/Reuters)

Oklahoma may have used the wrong drug during an execution in January, just days before the U.S. Supreme Court agreed to hear a case challenging the state’s lethal-injection protocol, officials said Thursday.

The revelation comes a week after Oklahoma hastily called off another scheduled lethal injection because corrections officials said they realized they had the wrong drug for that execution. And it comes during a period of heightened focus on capital punishment in the United States, with much of the spotlight repeatedly landing on Oklahoma, which drew international attention for bungling an execution last year. 

After Oklahoma executed Charles Warner in January, an autopsy report said that his body and containers for lethal-injection drugs were delivered to the state’s Office of the Chief Medical Examiner. Among those containers were empty vials labeled potassium acetate, even though the state’s lethal-injection protocol calls for the use of potassium chloride. (There were also empty syringes labeled potassium chloride.)

The autopsy report’s findings were first reported by the Oklahoman newspaper. A copy of the report was sent from the medical examiner’s office to The Washington Post and other news organizations Thursday.

[The death penalty continued its slow, steady decline last year]

Oklahoma’s lethal-injection protocol lists a trio of drugs that can be used for a three-drug lethal injection: midazolam, a sedative; vecuronium bromide, a paralytic; and potassium chloride, which stops the heart. The protocol does not include potassium acetate.

Last week, the scheduled execution of Richard Glossip was delayed and then stayed indefinitely when authorities said they opened the sealed box of drugs and found potassium acetate rather than potassium chloride.

Gov. Mary Fallin (R) said she first learned last week, while officials were discussing delaying Glossip’s execution, that the state may have used potassium acetate to execute Warner.

“As an act of precaution, the attorney general and I decided to stop the execution,” Fallin said in a statement. “During the discussion of the delay of the execution it became apparent that [Department of Corrections] may have used potassium acetate in the execution of Charles Warner in January of this year.”

FILE - This June 29, 2011, photo provided by the Oklahoma Department of Corrections shows Charles Warner, who was executed on Jan. 15, 2015 for the 1997 killing of his roommate's 11-month-old daughter. The Oklahoman reported Thursday, Oct. 8, 2015 that corrections officials used potassium acetate, not potassium chloride, as required under the state's protocol, to execute Warner.(AP Photo/Oklahoma Department of Corrections, File) Charles Warner in a 2011 photo. (Oklahoma Department of Corrections via AP)

After the wrong drug was found last week, Scott Pruitt (R), the Oklahoma attorney general, asked the state Court of Criminal Appeals to indefinitely stay that execution and two others originally scheduled for this month while his office investigated what happened before Glossip’s planned lethal injection. The court granted the request last Friday.

[Glossip’s execution was originally one of six set to occur in U.S. during a nine-day span, but three were halted]

Pruitt’s investigation will also include any activity “relevant to the use of potassium acetate and potassium chloride,” which will include things that occurred before Glossip’s halted execution, he said in a statement Thursday.

Oklahoma “has a strong interest in ensuring that the execution protocol is strictly followed,” he said.

Robert Patton, director of the Oklahoma Department of Corrections, told reporters last week that the provider of the drugs said potassium acetate and potassium chloride are “medically interchangeable.” But he said he asked Fallin to call off the execution because of the uncertainty regarding the drug.

The two different types of potassium appear to be equal, said David Gortler, an associate professor of pharmacology at Yale University and a drug safety expert.

“Potassium is potassium,” Gortler said. “The pharmacology isn’t that much different between the different salts.”

Opponents of Oklahoma’s death penalty called Thursday for more information about what happened. Charles J. Ogletree Jr., a professor at Harvard Law School and prominent critic of the death penalty, said the Justice Department should investigate the situation.

Dale Baich, an attorney for Glossip and Warner, vowed to pursue the matter through the courts.

“We cannot trust Oklahoma to get it right or to tell the truth,” Baich said in a statement. He said Oklahoma’s acknowledgment that it may have used the wrong drug “yet again raises serious questions about the ability of the Oklahoma Department of Corrections to carry out executions.”

Connie Johnson, chair of the Oklahoma Coalition to Abolish the Death Penalty, said in a statement that news about the potassium acetate “reinforces our efforts and renews our determination to continue to educate Oklahomans about the many things that are wrong with the death penalty.”

[Nebraska group says it may have enough signatures to suspend the state’s repeal of the death penalty]

Fallin said her office supports Pruitt’s investigation and added that she hopes it will “make sure justice is served competently and fairly.”

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

The Oklahoma Department of Corrections declined to comment Thursday while the investigation is ongoing.

“As you are aware the Attorney General’s office has opened an inquiry,” Patton said in an e-mailed statement Thursday. “Out of respect for that inquiry we will not making any comment at this time.”

Oklahoma went to the Supreme Court earlier this year to defend its execution protocol, which was adopted last year following the botched execution of Clayton Lockett.

Lockett, who was convicted of murder and other charges, grimaced, bucked his body and kicked during his execution. Warner, who was convicted of raping and murdering an 11-month-old, had been scheduled to die the same night as Lockett, but his lethal injection was postponed for months as the state investigated Lockett’s execution.

The bungled lethal injection of Lockett, one of three instances last year that an execution went awry in the United States, drew criticism from death penalty opponentsPresident Obama and the United Nations. A state investigation found that the execution team failed to properly place the intravenous needle to deliver the lethal injection drugs to Lockett.

Warner’s autopsy report states that his probable cause of death was “judicial execution by lethal injection.” In January, he became the first — and, so far, only — inmate executed using the state’s new protocol. Media witnesses reported that Warner did not show any signs of distress during the execution, but the Associated Press reported that after the midazolam was administered, he said, “My body is on fire.”

The following week, the Supreme Court said it would hear the challenge to Oklahoma’s protocol, a case that largely focused on the sedative midazolam, which was also used in the two other problematic executions last year. An Arizona execution lasted for nearly two hours as the inmate gasped and snorted before dying, while an Ohio execution lasted for nearly 25 minutes as the inmate appeared to gasp and choke. Ohio now says it will no longer use midazolam. (Florida has also used midazolam repeatedly without witnesses reporting these kinds of issues.)

[Most Americans support the death penalty. They also agree that an innocent person might get put to death.]

A majority of Americans support the death penalty, and while that support has fallen since the 1990s, it did not shift after last year’s highly publicized problems.

Meanwhile, though fewer executions are taking place in the United States, lethal injection remains the country’s primary method of execution. In recent years, states have had trouble getting the necessary drugs, largely because European companies and officials have protested the use of drugs supplied by European companies in American executions.

Most lethal injections in the country had, until very recently, been carried out using a three-drug combination. But states have been scrambling amid the drug shortage, shifting lethal-injection protocols again and again. Some states have been using a single drug, including the two most active death penalty states since 2013, Texas and Missouri, both of which use only the anesthetic pentobarbital. (Here is a guide to the drugs being used across the country.)

As the uncertainty regarding drugs persisted, some states debated other ways to execute inmates, and three states made some of these changes: Oklahoma adopted nitrogen gas as a new backup method of execution earlier this year, while Utah revived firing squads and Tennessee expanded its ability to use the electric chair.

Oklahoma’s lethal-injection procedure was upheld by the Supreme Court in June. Experts said there did not immediately appear to be a nationwide scramble for midazolam following that ruling.

The justices used the Oklahoma case, which bore Glossip’s name, as a forum to debate the overall issue of capital punishment. Justice Stephen G. Breyer, joined by Justice Ruth Bader Ginsburg, wrote a lengthy dissent questioning whether the death penalty was constitutional. Justice Antonin Scalia, joined by Justice Clarence Thomas, dismissed this as “gobbledy-gook.”

After that ruling came down, Oklahoma vowed to resume executions and Glossip’s lethal injection was scheduled for Sept. 16. He came within hours of being executed when a state appeals court called off the lethal injection to review his appeals. The same court later rejected the appeals, and his execution was set for Sept. 30.

That day, his scheduled execution time came and went, but Fallin eventually issued an unexpected stay to allow the state to make sure it was “complying fully with the protocols approved by federal courts.” His execution is now on hold indefinitely.

Related:

What it was like watching the botched Oklahoma execution last year

Innocent man who spent 30 years on death row died hours before Supreme Court justices cited him

Lethal injection has become a fractured process across the United States

Connecticut Supreme Court says the death penalty is unconstitutional and bans executions for inmates on death row

The state Supreme Court justice who stepped down to protest the death penalty

This post has been updated.