The former F.B.I. agent at the heart of a grand jury investigation into the chief prosecutor of St. Louis has been indicted on seven felony counts of perjury and tampering with evidence, according to documents unsealed on Monday.

The former agent, William D. Tisaby, was hired last year by the city’s prosecutor, Kimberly Gardner, to help investigate accusations against the governor of Missouri at the time, Eric Greitens. Mr. Greitens was accused of threatening to blackmail his mistress with a photo of her partially clothed if she ever revealed their affair.

Mr. Tisaby interviewed the woman in January 2018 with Ms. Gardner present. During a sworn deposition two months later, conducted by Mr. Greitens’s lawyers, Mr. Tisaby made false statements, including that he had not made any notes during the interview and that a video recorder had malfunctioned.

Ms. Gardner had indicted Mr. Greitens for felony invasion of privacy. But Mr. Tisaby’s problematic deposition testimony played a role in the dismissal of the case in May 2018. Lawyers for Mr. Greitens then filed a complaint accusing Mr. Tisaby of perjury with the St. Louis police. A special prosecutor was appointed to investigate.