The scandal that brought down CIA Director David Petraeus started with harassing emails sent by his biographer and paramour, Paula Broadwell, to another woman, and eventually led the FBI to discover the affair, U.S. officials told The Associated Press on Saturday.
Petraeus quit Friday after acknowledging an extramarital relationship.
The official said the FBI investigation began several months ago with a complaint against Broadwell, a 40-year-old graduate of the U.S. Military Academy and an Army Reserve officer. That probe led agents to her email account, which uncovered the relationship with the 60-year-old retired four-star general, who earned acclaim for his leadership of the wars in Iraq and Afghanistan.
The identity of the other woman and her connection with Broadwell were not immediately known.
Petraeus has been married for 38 years to Holly Petraeus, the daughter of the West Point superintendent when he was a student at the New York school.
Concerned that the emails he exchanged with Broadwell raised the possibility of a security breach, the FBI brought the matter up with Petraeus directly, according to the official, who spoke on condition of anonymity because he was not authorized to publicly discuss the investigation. The FBI approached the CIA director because his emails in the matter were in most instances sent from a personal account, not his CIA one.
Petraeus decided to quit, abruptly ending a high-profile career that might have culminated with a run for the presidency, a notion he was believed to be considering. "Such behavior is unacceptable, both as a husband and as the leader of an organization such as ours," Petraeus wrote his staff.
Petraeus handed his resignation letter to President Barack Obama on Thursday, stunning many in the White House, the CIA and Congress. The news broke in the media before the House and Senate intelligence committees had been briefed, officials say.
By Friday evening, multiple officials identified Broadwell, who spent the better part of a year reporting on Petraeus' time in Afghanistan.
Her best-selling biography, "All In: The Education of General David Petraeus," was written with Vernon Loeb, a Washington Post editor, and published in January.
Since Petraeus's resignation on Friday, the book jumped from a ranking on Amazon of 76,792 on Friday to 111 by mid-Saturday.
The CIA was not commenting on the identity of the woman with whom Petraeus was involved.
Broadwell, who is married with two young sons, has not responded to multiple emails and phone messages. Broadwell planned to celebrate her 40th birthday party in Washington this weekend, with many reporters invited.