Republican Tennessee Gov. Bill Lee has paused all executions in his state for the rest of 2022, announcing an investigation Monday into an "oversight" regarding the state's lethal injection process.

Lee granted a last-minute delay to death row inmate Oscar Franklin Smith last week, who was scheduled to be executed April 21. The state said there had been an oversight in the lethal injection process, leading to the formula only being tested for lethality and sterility, but not for endotoxins.

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Lee has now ordered a third-party review of the state's lethal injection process, retaining former U.S. Attorney Ed Stanton to conduct the investigation.

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"I review each death penalty case and believe it is an appropriate punishment for heinous crimes," Lee said in a statement. "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."

Smith's execution was one of five scheduled to take place this year. Death row inmates in the state have the option to request execution by electric chair rather than lethal injection, which is the default. Three of four executions in Tennessee have been carried out via electric chair since 2019, according to the governor's office.

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Smith was sentenced to death for fatally stabbing and shooting his wife, Judith Smith, and her two sons, Jason and Chad Burnett, in 1989.

Both Lee and the U.S. Supreme Court rejected appeals from Smith prior to his originally-scheduled April 21 execution. Lee granted the pause hours before the execution was to take place.