Jason Locke stepped down as interim leader in June amid the investigation.
Locke had been a finalist for the permanent post, but Gov. Bill Haslam's office produced emails that Haslam received from Locke's wife. The emails alleged her husband used taxpayer money to carry on an affair with another state official, Sejal West of the Tennessee Department of Mental Health & Substance Abuse Services.
The grand jury also said it found no violation of law by West.
Locke's lawyer, Jeff Cherry, issued a statement Friday evening saying Locke cooperated throughout the investigation and chose to retire from the TBI out of respect for the agency and not because he was guilty of a crime.
The Davidson County grand jury foreman's statement dated Tuesday was released by the district attorney's office on Friday.
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