Email investigation led to discovery of Petraeus' affair

(CBS News) We're learning more about the investigation that lead to the abrupt resignation of CIA chief and retired four-star general, David Petraeus. He admitted to an affair and submitted his resignation to President Obama. And now it turns out it was the FBI that discovered the affair was with his biographer.

Petraeus is one of America's most decorated and revered generals. As head of the CIA, he was in charge of the nation's most sensitive operations. But, the rigidly-disciplined Petraeus was hiding his own dark secret.

Sources say CIA director Petraeus was never the target of an FBI investigation. But when his name surfaced in another probe, agents became concerned Petraeus or his email accounts may have been compromised.

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CIA Director David Petraeus resigns

It all began a few months ago when a female acquaintance of the CIA director received a series of harassing emails, which apparently referenced Petraeus. The woman, who sources say is not a government official, notified the FBI.

Federal agents quickly traced the emails back to Petraeus' biographer, Paula Broadwell. A further investigation then revealed evidence of an extra-marital affair, including numerous cryptic emails between the retired four-star general and Broadwell.

Broadwell spent time with Petraeus when he commanded forces in Afghanistan, doing research for her book, "All In: The Education of General David Petraeus."

On a publicity tour last February, the married mother of two children made this comment.

"I'm not in love with David Petraeus, but I think he does present a terrific role model for young people, for executives, for men and women, no matter what."

Petraeus, who is a also a married father of two, publicly praised his wife, Holly, during his Senate confirmation hearing: "She is an army daughter, an army wife, an army mother, and an advocate for military families. I've been blessed to have her in my corner for 37 years and 23 moves, and I appreciate the opportunity this afternoon to recognize her publicly. "

But in his resignation message to the CIA workforce, Petraeus confessed a moral failing, saying he used "...extremely poor judgment..." which was "...unacceptable, both as a husband and as the leader of an organization such as ours."

The FBI investigation is now winding down. There is no evidence that any sensitive CIA information was ever compromised, and no one is expected to face any criminal charges.