CIA Director David Petraeus and his biographer, Paula Broadwell, used a joint email account in order to exchange messages, the Associated Press reports. Instead of sending emails, they would save draft messages in the draft folder of the same account -- a trick famous for being used by terrorists.
From the AP:
FBI agents traced the alleged cyber harassment to Broadwell, the officials said, and discovered she was exchanging intimate messages with a private Gmail account. Further investigation revealed the account belonged to Petraeus under an alias.
Petraeus and Broadwell apparently used a trick, known to terrorists and teenagers alike, to conceal their email traffic, one of the law enforcement officials said.
Rather than transmitting emails to the other's inbox, they composed at least some messages and instead of transmitting them, left them in a draft folder or in an electronic "dropbox," the official said. Then the other person could log onto the same account and read the draft emails there. This avoids creating an email trail that is easier to trace.