The head of Oklahoma's prison system, who presided over two botched lethal injections and a third that was called off because the wrong drug was delivered, announced his resignation Friday amid an investigation into what went wrong with the executions.

Department of Corrections Director Robert Patton said in a news release that he will resign effective Jan. 31, but will begin taking accrued leave on Dec. 25. He said he has accepted a position in Arizona to be closer to family.

Patton didn't immediately respond to a telephone message left at his office, and department spokeswoman Terri Watkins said she didn't have any information about his new position. But Watkins said Patton's resignation was not connected to the investigation by Attorney General Scott Pruitt.

"It's to go spend more time with his five grandchildren in Arizona," Watkins said.

Patton had been on the job a few months in April 2014 when a botched lethal injection left inmate Clayton Lockett writhing on the gurney and mumbling in an execution that Patton tried unsuccessfully to halt before Lockett was pronounced dead 43 minutes after the procedure began.

An investigation later revealed that a faulty insertion of the intravenous line and lack of training of the execution team contributed to the problems.

In September, Richard Glossip was just hours away from his scheduled execution when prison officials realized they received potassium acetate, not potassium chloride, which is the third of three drugs the state uses to execute people. After Glossip's execution was put on hold, an autopsy report from Charles Warner's January execution revealed he was administered potassium acetate instead of potassium chloride.

Both Patton and Oklahoma State Penitentiary warden Anita Trammell appeared in October before a multicounty grand jury that is investigating how the wrong lethal injection drugs were used during Warner's execution. Trammell announced her retirement just days after that appearance.

Gov. Mary Fallin issued a statement Friday saying she appreciated Patton's efforts to keep prisons safe and embrace reform efforts to emphasize rehabilitation and treatment for nonviolent offenders. "I regret his departure," Fallin said.

But Fallin had declined to publicly endorse Patton after problems with the latest scheduled execution, even after he was given a vote of confidence by the Board of Corrections.

Pruitt, through a spokesman, declined to comment on Patton's resignation or his investigation. At Pruitt's request, the Oklahoma Court of Criminal Appeals has issued an indefinite stay of all scheduled executions.

The attorney general has said he won't request any execution dates until at least 150 days after his investigation is complete, the results are made public and his office receives notice that the prisons agency can comply with the state's execution protocol.