The extramarital affair between CIA Director Gen. David Petraeus and his biographer Paula Broadwell was exposed as the result of FBI e-mail monitoring of Broadwell after she sent "threatening and harassing" messages to another woman. The Washington Post reports that the months-long investigation's discovery of explicit, sexually charged emails from Petraeus' personal e-mail account to Broadwell led directly to Petraeus' resignation.

The unnamed woman, which the Washington Post says did not work at the CIA and was not Petraeus' wife, had asked the FBI to look into the threatening e-mails from Broadwell. FBI investigators then used the bureau's electronic monitoring capability, (as described in depth in Ars' analysis of the FBI's electronic monitoring of Fort Hood shooter Nidal Hasan), to intercept emails to and from Broadwell's account.

During the course of the investigation, federal agents monitoring Broadwell's e-mails found messages coming from Petraeus' personal Gmail account, and were concerned that his account had been hacked, "leading to concerns about potential national security breaches," according to officials interviewed by the Post. But after collecting more evidence, the FBI realized it had uncovered an affair between Petraeus and Broadwell. The agency later decided that Broadwell's e-mails to the unnamed woman were "'threatening and harassing' but not specific enough to warrant criminal charges."