I have to say I’m out of the loop.
What was the criminal act the FBI was investigating?
Was it the Affair of Petreaus that is Criminal?
Was it the cat fight!
IDK?????
Cyberstalking. See 18 USC 875(c) and/or 47 USC 223. Cyber-Stalking: Federal Cyberstalking Laws
Kelley told a friend, who happens to be an FBI agent, about the anonymous e-mails she was getting, telling her to "lay off" General Allen. While I would hazard a guess that most cyberstalking reports go un-acted on, this report got special attention, on account of the friendship Kelley had or has with the FBI agent.
Petraeus was not a target of the investigation. The point of the investigation was to learn who was sending the anonymous e-mails to Kelley. That anonymous person was Broadwell.