NASHVILLE (AP) — Gov. Bill Lee paused executions in Tennessee for the rest of the year on Monday after revealing that the state had failed to ensure its lethal injection drugs were properly tested. The oversight forced Lee to abruptly halt the execution of Oscar Smith an hour before he was to die last month.

Lee didn't initially disclose the reason for stopping Smith's execution other than to say there was an "oversight" in the preparation of the lethal injection drugs. Tennessee's execution protocols require any compounded drugs to be independently tested for potency, sterility and endotoxins. In his Monday statement, Lee said the drugs for Smith's execution were tested for potency and sterility, but not endotoxins.

Smith's attorneys had called for a moratorium on executions and independent review of the problems last week. In a Monday statement, Federal Public Defender Kelley Henry said the Republican governor's decision shows "great leadership."

"The use of compounded drugs in the context of lethal injection is fraught with risk," Henry said. "The failure to test for endotoxins is a violation of the protocol. Governor Lee did the right thing by stopping executions because of this breach."

Lee appointed former U.S. Attorney Ed Stanton to review circumstances that led to the failure. He'll also review the clarity of the state's lethal injection manual and look at Tennessee Department of Correction staffing, Lee said in a statement.

"I review each death penalty case and believe it is an appropriate punishment for heinous crimes," Lee said. "However, the death penalty is an extremely serious matter, and I expect the Tennessee Department of Correction to leave no question that procedures are correctly followed."

The pause will remain in effect through the end of the year to allow time for the review and corrective action, Lee said.

Many states turned to compounded drugs after commercial drug manufacturers began refusing to sell their medications for executions, making the drugs difficult for prison systems to obtain.

Frank Romanelli, professor of pharmacy at the University of Kentucky College of Pharmacy, explained the difference like this: "I can go buy a Hershey's chocolate bar, or I can make one at home. They are trying to make one at home, and they have to do testing to check that it is equivalent."

The presence of endotoxins, which usually come from bacteria, could be an indication of problems with the manufacture of the drugs, but the endotoxins themselves likely wouldn't cause a problem in an execution setting, Romanelli said. That's because endotoxins typically are not immediately fatal.

The testing would be done normally by a pharmacy that produces the drugs for a medical setting, and Romanelli suspects Tennessee lifted those guidelines for its execution protocols.

"In their defense, they are probably thinking, 'Let's keep this as clean as possible — as airtight as possible,'" Romanelli said.

Henry said last week that the night before the execution, she requested the results of the testing but received no response. Henry believes at least two of the three execution drugs were compounded, rather than commercially manufactured, she said, although secrecy rules surrounding Tennessee executions makes it difficult to know for certain.

Tennessee and many other states have passed exemptions to open records laws in recent years, shrouding the identity of drug suppliers and other information about executions in secrecy.

It was through a public records request that Henry received heavily redacted records from the state's last lethal injection execution in 2019. She believes the drugs in that case did not pass the required tests. Drugs are supposed to be tested to United States Pharmacopeial Convention (USP) standards. Henry said she believes those drugs were tested but using a standard from a different country, likely because they were sourced from overseas.

Lee's statement said he has asked Stanton to review the "adherence to testing policies" since the state's lethal injection manual was last updated in 2018.

While lethal injection was adopted as a humane alternative to the electric chair, it has been the subject of consistent problems and lawsuits.

"Every single time people at my office have raised problems, they've had to make adjustments — whether it's the wrong drug, the wrong potency, or the USP," Henry said at a news conference last week.

Tennessee uses a three-drug series to put inmates to death: midazolam, a sedative to render the inmate unconscious; vecuronium bromide, to paralyze the inmate; and potassium chloride, to stop the heart. Officials have said the inmates are unconscious and unable to feel pain. Expert witnesses for inmates, however, have said the inmates would feel like they are drowning, suffocating and being burned alive, all while unable to move or call out.

Of the seven inmates Tennessee has put to death since 2018 — when Tennessee ended an execution pause stretching back to 2009 — five have chosen to die in the electric chair. Smith, 72, declined to make a choice, meaning he was scheduled to be executed by the state's preferred method of lethal injection.

Smith was sentenced to death for fatally stabbing and shooting his estranged wife, Judith Smith, and her teenage sons, Jason and Chad Burnett, at their Nashville home on Oct. 1, 1989.

Smith's execution was to be the first of five scheduled in 2022, the most of any state other than Texas, which also scheduled five executions, according to the Death Penalty Information Center. It was also to be Tennessee's first execution since the pandemic halted executions in 2020.

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Reynolds reported from Louisville, Kentucky