Officials say FBI probe of Petraeus began with 'suspicious emails'
But the CIA director's name unexpectedly turned up in the course of the investigation, two officials and two other sources briefed on the matter said.
It was "an issue with two women and they stumbled across the affair with Petraeus," a U.S. government security source said.
The Washington Post reported on Saturday that the FBI probe was triggered when Broadwell sent threatening emails to an unidentified woman close to the CIA director.
The woman went to the FBI, which traced the threats to Broadwell and then uncovered explicit emails between Petraeus and Broadwell, the Post said.
Attempts by Reuters and other news media to reach Broadwell, an Army reserve offer and author of a biography of Petraeus, have not been successful.
The FBI and CIA declined comment on Saturday.
Many questions in the case remain unanswered publicly, including the identity of the second woman; the precise nature of the emails that launched the FBI investigation; and whether U.S. security was compromised in any way.
Nor is it clear why the FBI waited until Election Day to tell U.S. Director of National Intelligence James Clapper, who oversees the CIA and other intelligence agencies, about its investigation involving Petraeus.
The CIA director announced his resignation suddenly on Friday, acknowledging an extramarital affair and saying he showed "extremely poor judgment."
The developments likely ended the public career of one of the United States' most highly regarded generals, who was credited with helping pull Iraq out of civil war and led U.S. and NATO troops in Afghanistan.
Meanwhile, new details emerged on Saturday about developments in the final days leading to Petraeus' departure from atop the CIA.
Clapper was notified by the FBI on Tuesday evening about 5 p.m. - just as returns in the U.S. presidential election were about to come in - about "the situation involving Director Petraeus," a senior intelligence official said. Clapper and Petraeus then spoke that evening and the following morning.
"Director Clapper, as a friend and a colleague and a fellow general officer, advised Director Petraeus that he should do the right thing and he should step down," the official said.
Clapper is a retired Air Force lieutenant general; Petraeus served nearly four decades in the U.S. Army.
On Wednesday, Clapper notified the National Security Council at the White House that Petraeus was considering resigning and President Barack Obama should be informed, the official said.
Law enforcement and intelligence officials agreed to discuss the Petraeus matter only on condition of anonymity because of the issue's sensitivity and because it is the subject of a law enforcement investigation.
Once Petraeus' name turned up in the investigation, the importance of the FBI inquiry was immediately escalated, as investigators became concerned the CIA chief somehow might have been compromised, the law enforcement official said.
However, the official and two sources briefed on the matter said no evidence has turned up suggesting Petraeus had become vulnerable to espionage or blackmail.
At this point, it appears unlikely that anyone will be charged with a crime as a result of the investigation, the official said.
The FBI investigation began fairly recently - months ago rather than years ago, when Petraeus would still have been in uniform as one of the U.S. Army's top field commanders, the official said.
Representative Peter King, Republican chairman of the House of Representatives' Homeland Security Committee, said in an interview on MSNBC the FBI was "investigating or monitoring ... the director of the CIA for four or five months."
Several officials briefed on the matter said senior officials at the Pentagon, CIA and Congress knew nothing of the FBI's investigation of Petraeus until Thursday afternoon at the earliest, and some key officials were not briefed on the details until Friday.
There is no evidence at this time that anyone at the White House had knowledge of the situation involving Petraeus prior to the U.S. presidential election on Tuesday, which saw Obama elected to a second four-year term.
Another U.S. government security source said it was not until Friday afternoon that some members of the House and Senate intelligence oversight committees were notified about Petraeus' resignation by Clapper's office.
The congressional committees were told that it was a personal issue that Petraeus had to discuss with his wife. When pressed, a representative of the Office of the Director of National Intelligence said it involved another woman.