Petraeus stepped down as D/CIA because of an affair with his biographer Broadwell, who had sent anonymous threatening email to another woman Kelly.  Ms. Kelly contacted an acquaintance in the FBI who kicked it over to the cybercrime division.

The FBI, finding neither legal wrongdoing nor actionable security breaches, closed the investigation. However, Ms. Kelly's contact in the FBI, though previously warned off the case, assumed the investigation had become a cover-up and went tattling to House Rep. Eric Cantor.

Who was this mysterious FBI agent?  Ms. Kelly knew him since he'd sent her some shirtless photos of himself.

So, at beginning and end, the whole investigation and scandal was unwittingly moved by someone who simply wanted to screw the alleged victim.

FBI Agent 'X' is currently under investigation by the FBI internal affairs office.