In May this year, Jill Kelley, an unpaid social liaison for the military in Tampa, tipped off the FBI about receiving threatening emails from an anonymous sender, warning her to keep her hands off CIA Chief David Petraeus. The tip-off sparked a probe into the CIA boss.

The FBI traced the emails to an account owned by Petraeus' mistress, Paula Broadwell, exposing his affair and leading to his resignation.

Broadwell, an army veteran, had met Petraeus in 2006, right before conducting a three-year research project to write a biography about him, entitled "All In." Their affair reportedly took off last year.

The reason this scandal is so explosive, is that the man at the centre of the storm is David Petraeus, the most celebrated U.S. general of his generation.

Credited with turning around the war in Iraq in 2007, he became the commander of U.S. forces in Afghanistan in 2010 and there was even talk of a possible run for presidency before he became CIA chief in September 2011.

The White House only learned of the probe on November 6 - Election Day - although it began months earlier. Congressional lawmakers complained that they should have been informed earlier about the probe as it shook up the entire intelligence and military establishment.

In a dramatic new twist in the Petraeus sex scandal on Oct. 12, it emerged that General John Allen had exchanged 20,000 to 30,000 messages - as many as 40 a day - with Jill Kelley.

Allen, 58, succeeded Petraeus in Afghanistan and was due to take over as NATO's top commander, but his appointment is now on hold as the Pentagon investigates Allen's relationship with Kelley.