WASHINGTON -- 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 said Saturday.
Mr. Petraeus quit Friday after acknowledging an extramarital relationship.
The officials said the FBI investigation began several months ago with a complaint against Ms. 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 Army general, who earned acclaim for his leadership of the wars in Iraq and Afghanistan.
The identity of the other woman and her connection with Ms. Broadwell were not immediately known.
Ms. Petraeus has been married for 38 years to Holly Petraeus, the daughter of the West Point superintendent when he was a student at the U.S. Military Academy.
Members of Congress said they want answers to questions about the affair that led to Mr. Petraeus's resignation.
The House intelligence committee chairman, Mike Rogers, R-Mich., and ranking member Dutch Ruppersberger, D-Md., will meet Wednesday with FBI deputy director Sean Joyce, and CIA acting director Michael Morell to ask questions, including how the investigation came about, according to a senior congressional staffer who spoke anonymously because he was not authorized to discuss the investigation publicly.
Concerned that the emails he exchanged with Ms. Broadwell raised the possibility of a security breach, the FBI brought the matter up with Mr. Petraeus directly, according to an federal 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 in most instances were sent from a personal account, not his CIA one.
Mr. 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 considering.
"Such behavior is unacceptable, both as a husband and as the leader of an organization such as ours," Mr. Petraeus wrote his staff.
Mr. 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 were briefed, officials say.
By Friday evening, multiple officials identified Ms. Broadwell, who spent the better part of a year reporting on Mr. 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 Mr. Petraeus' resignation on Friday, the book jumped from a ranking on Amazon of 76,792 on Friday to 111 by mid-Saturday.
Ms. Broadwell, who is married with two young sons, has not responded to multiple emails and phone messages. Ms. Broadwell planned to celebrate her 40th birthday with a party in Washington this weekend. Many reporters were invited. But her husband emailed guests to cancel the event late Friday.
CIA officers long had expressed concern about Ms. Broadwell's unprecedented access to the director. She frequently visited the spy agency's headquarters in Langley, Va., to meet Mr. Petraeus in his office, accompanied him on his punishing morning runs around the CIA grounds and often attended public functions as his guest, according to two former intelligence officials.
As a military intelligence officer in the Army Reserve, Ms. Broadwell had a high security clearance, which she mentioned at public events as one of the reasons she was well-suited to write Mr. Petraeus' story.
But her access was unsettling to members of the secretive and compartmentalized intelligence agency, where husbands and wives often work in different divisions, but share nothing with each other when they come home because they don't "need to know."
Mr. Petraeus' staff in Afghanistan similarly had been concerned about the time Ms. Broadwell spent with their boss on her multiple reporting visits to the war zone. Following standard military procedure with senior officers, they always had another staffer present when she met with him at his headquarters. Military officers close to him insist the affair did not begin when he was in uniform.
The FBI investigators were not pursuing evidence of Mr. Petraeus' marital infidelity, which would not be a criminal matter, the official said. But their examination of his emails, most or all of them sent from a personal account and not from his CIA account, raised the possibility of security breaches that needed to be addressed directly with him.
"Alarms went off on larger security issues," the official said. As a result, FBI agents spoke with the CIA director about two weeks ago, and he learned in the discussion, if he was not already aware, that they knew of his affair with Ms. Broadwell, the official said.
But the fears of bigger security problems proved unjustified, and the security questions were resolved, the official said.
In the preface to her book, Ms. Broadwell said she first met Mr. Petraeus in the spring of 2006. She was a graduate student at the Kennedy School of Government at Harvard; he was visiting the university to discuss his experiences in Iraq and a new counterinsurgency manual he was working on.
Harvard invited some students to meet with Mr. Petraeus, and Ms. Broadwell was among them because of her military background, which she wrote included being recalled to active duty three times to work on counterterrorism issues after the Sept. 11, 2001, attacks.
After Mr. Obama put Mr. Petraeus in charge in Afghanistan in 2010, Ms. Broadwell decided to expand her research into an authorized biography.
The New York Times contributed.