By Breaking Celebrity News Staff

The stunning revelations about CIA Chief David Petraeus' extramarital affair came about because a woman close to him began receiving anonymous threatening emails so frightening she went to the FBI for help.

The investigating agents were in for a shock when they tracked the scary messages to Petraeus' own biographer, Paula Broadwell, The Washington Post is reporting.

Then combing through Broadwell's other email correspondences, the Feds unwittingly stumbled into Broadwell's affair with the retired four star general and a trail of explicit messages between the two of them, including ones about "sex under the desk."

The identity of the new "other woman" has not yet been revealed, but it was not Petraeus' wife Holly and was not anyone who worked for Petraeus at the CIA.

The Post's multiple sources would only say that Broadwell's messages indicated she felt the mystery woman was a threat to her own relationship with Petraeus.

As Breaking Celebrity News has been reporting, Petraeus announced on Friday that he had resigned as CIA and admitted to an extramarital affair.

“After being married for over 37 years, I showed extremely poor judgment by engaging in an extramarital affair,” Petraeus said in his statement. “Such behavior is unacceptable, both as a husband and as the leader of an organization such as ours. This afternoon, the president graciously accepted my resignation.”

His mistress, Paula Broadwell, is a married mother of two and a self-pro