Gen. Petraeus quits just DAYS before he was due to testify at Benghazi Libya Terrorist Attack Hearings

CIA Director David Petraeus resigned Friday after admitting to an extramarital affair -- an affair with his biographer that was revealed over the course of an FBI investigation, Fox News has learned. 

The FBI had been investigating an unrelated and much broader case before stumbling on the affair. Fox News has learned that during the course of this investigation, the name of biographer Paula Broadwell came up. The FBI followed that lead and in doing so, uncovered his affair with her. The FBI for some time was concerned that perhaps Petraeus was some sort of victim, but there has been no evidence discovered to back up such concerns.

Broadwell co-wrote Petraeus' biography, "All In." 

Petraeus met with President Obama on Thursday before submitting his letter of resignation, which the president accepted.  In a message to staff, Petraeus said he asked "to be allowed" to step down. 

"After being married for over 37 years, I showed extremely poor judgment by engaging in an extramarital affair. Such behavior is unacceptable, both as a husband and as the leader of an organization such as ours," the retired four-star general said. "This afternoon, the President graciously accepted my resignation."

The move comes amid the unfolding controversy surrounding the attack on the U.S. Consulate in Libya. Scrutiny has fallen on a range of agencies including the CIA, and the director had been set to testify at hearings next week -- he is no longer expected to do so. But Petraeus, in his resignation message, cited strictly "personal reasons" surrounding the affair. 

Petraeus' wife, Holly, also works in the Obama administration, for the Consumer Financial Protection Bureau. 

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