Mr. Greitens admitted the affair, but said he never hit her, coerced her or threatened blackmail.
The encounter occurred in the jurisdiction of Ms. Gardner, officially known as the circuit attorney for St. Louis. She hired the former F.B.I. agent, William Tisaby, to interview the hairdresser. In February 2018, she indicted Mr. Greitens on a charge of felony invasion of privacy.
During a sworn deposition the following month, though, Mr. Tisaby made a number of false statements, including that he did not have notes from the interview, and that a video recorder had malfunctioned.
But there were notes, and a videotape, both of which Ms. Gardner turned over to the Greitens legal team three weeks later. Armed with those, the defense lawyers went on offense.
They said the tape had been deliberately concealed because it showed the hairdresser contradicting what she had told a state legislative committee, undermining the case against Mr. Greitens.
And they accused Ms. Gardner, who had attended the interview and the deposition during which Mr. Tisaby made false statements, of having known about his notes and suborning his perjury.
“Nothing is more dangerous than a dishonest prosecutor,” one of Mr. Greitens’s lawyers, James Martin, told a judge last year, calling Ms. Gardner “a full participant in this conspiracy to keep information from us.”
Failure to turn over the tape would be a far more serious concern if it had the potential to help exonerate the defendant. But it is not clear that the video would have: When the state legislative committee investigating the controversy, composed largely of Mr. Greitens’s fellow Republicans, reviewed the tape, it found that the hairdresser’s accounts had been consistent.