David Petraeus and his lover Paula Broadwell used a trick often employed by terrorists as they sought to keep their email exchanges secret.
The CIA director and his biographer had a shared Gmail account and wrote some of their personal missives as draft emails, which were left in a draft folder or an electronic, "dropbox".
They could each then access the shared account and open the other's draft messages, reading them without the emails actually being sent.
Sending emails between two separate accounts leaves an electronic trail, including IP addresses that have been used.
Al Qaeda terrorists began using the technique years ago. It allowed them to exchange information without sending traceable emails. An account would be opened by one conspirator and a draft email written and saved. The account could then be opened by a co-conspirator with access anywhere in the world, who could read the draft email and delete it.
The process meant the information had never been sent in the form of an email, making the electronic trail weaker. The technique has since been copied by teenagers in many countries to avoid having their emails monitored by their parents.
Despite the care taken by Gen Petraeus and Mrs Broadwell in keeping their email exchanges secret, it was precisely the type of trail they were hoping to avoid with the Gmail trick that resulted in their affair being exposed.
FBI investigators were able to use the data trail left when Jill Kelley, a 37-year-old Florida socialite who was family friends with Gen Petraeus, received emails allegedly warning her to stay away from the former CIA director. The data trail revealed that the emails were being sent by Mrs Broadwell from an anonymous email account, information which eventually brought the affair to light.
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