(CNN) -- Retired Army Gen. David Petraeus resigned from his CIA director post last week after an FBI investigation revealed he had an extramarital affair, an investigation that also prompted questions about whether his paramour had inappropriate access to classified information. The scandal also has sparked an investigation into whether Marine Corps Gen. John Allen, the top U.S. commander in Afghanistan, sent inappropriate messages to a different woman, leading President Obama to put Allen's nomination to become NATO's supreme allied chief on hold. The FBI uncovered the Petraeus affair while it investigated e-mails that his paramour, Paula Broadwell, allegedly sent to a Petraeus family friend, Jill Kelley, according to a U.S. official. Kelley, meanwhile, is the woman to whom Allen allegedly sent inappropriate e-mails, according to the Defense Department.
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NEW: Petraeus to testify Friday before congressional panel about Benghazi attack
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David Petraeus resigned as CIA chief last week after an investigation revealed an affair
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The scandal also sparked a probe into whether Gen. John Allen sent inappropriate e-mails
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Obama: I've seen no evidence of a potentially damaging national security breach
Paula Broadwell
It wasn't until two months later that the FBI told Kelley who had sent the e-mails, said the source, adding that Kelley does not know @placeholder and has never met her.
David Petraeus
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David Petraeus
Among those believed to be referenced in the e-mails was @placeholder.
Paula Broadwell
@placeholder's source for that previously unpublished information remains unclear, and there's no evidence that it came from Petraeus.
David Petraeus
Petraeus
Petraeus
Petraeus
David Petraeus
Broadwell's source for that previously unpublished information remains unclear, and there's no evidence that it came from @placeholder.
David Petraeus
Petraeus
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David Petraeus
He spoke to him via e-mail on Monday and is authorized by @placeholder to talk.
Marine Corps
John Allen
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John Allen
-- While the nature of the relationship between @placeholder and Kelley, if any, is unclear, evidence of an affair could subject the general to military prosecution.