EDIT: And here’s why Petraeus created a security risk by using Gmail in this manner.
It didn’t start with Petraeus, but in the course of the investigation they stumbled across him. We were stunned. …People think that because it’s the C.I.A. director, it must involve bigger issues. Think of a small circle of people who know each other.An anonymous Congressional official • Explaining to the New York Times, based on a briefing from the FBI, the lead-up to the bureau’s discovery of an affair between David Petraeus and his biographer, Paula Broadwell. The official said it was a complaint filed months ago regarding harassing emails sent to a woman (who they said to be not a government official or member of family) by Broadwell that drew the gaze of the FBI. When they ultimately gained access to her emails, some of those sent between her and Petraeus revealed they were having an affair. That in and of itself isn’t a legal issue, but the investigation ultimately raised security conerns regarding Petraeus’ email account. Petraeus resigned from his position as CIA director yesterday, citing “extremely poor judgment” in his extra-martial affair. source