FBI probe of Petraeus' emails purportedly led to discovery of extramarital affair

The FBI investigation that led to the discovery of CIA Director David Petraeus’ extramarital affair and his resignation Friday started when the agency began monitoring Petraeus’ email, Fox News has learned.

The agency was alerted that biographer Paula Broadwell, with whom Petraeus had the affair, may have had access to his personal email account.

The FBI investigation began when someone reported suspicious emails allegedly sent from Broadwell. The agency then determined that she allegedly had emailed a number of government employees. The FBI was at one point trying to determine whether any of the employees were being stalked, sources told Fox News.

The FBI investigation started with a complaint several months ago about “harassing” e-mails from Broadwell to an unidentified third person, a government official briefed on the case told The New York Times on Saturday.

Federal agents reportedly discovered the exchanges between Broadwell and Petraeus when following up on the complaint,  according to The Times' source, whom the newspaper said spoke on the condition of anonymity.

The person who complained about Broadwell's purported harassing messages was not a family member nor a government official.

A congressional official who was briefed on the matter Friday said senior intelligence officials had explained that the FBI. investigation “started with two women," the newspaper reported.

Broadwell told Fox News earlier this year when talking about the biography “All In: The Education of General David Petraeus” that she met the retired four-star Army general when she was post-graduate student at Harvard and he came to the university to speak.

They kept in touch via email and went running together when she came to Washington, Broadwell said on Don Imus’ Fox Business show.

“He gave me his card,” said Broadwell, who co-wrote the book. “We kept in touch.”

Broadwell also called Petraeus’ wife of 38 years, Holly Petraeus, “a wonderful military spouse.”

Source said the FBI investigation ended when the agency determined no criminal acts had been committed.

FBI spokesman Paul Bresson has declined to comment on the information that the affair had been discovered in the course of an investigation by the agency.

A senior staffer with the House Intelligence Committee tells Fox News that Chairman Mike Rogers, R-Mich. has been briefed on the resignation, adding that, "Rogers has serious questions and serious concerns about the resignation and circumstances surrounding it."

Sources sum up the sentiment about the situation as, "What the heck is going on at the agency?"

These sources tell Fox News that there is serious concern whether the chairman and ranking members of the committee were given appropriate notification by the FBI that Petraeus had surfaced within the scope of an investigation.

Petraeus, who turned 60 on Wednesday, met his wife when he was a cadet at the U.S. Military Academy at West Point, N.Y. She was the daughter of the academy superintendent. They have two children, and their son led an infantry platoon in Afghanistan.

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

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.

Obama, in a written statement, said Petraeus provided an "extraordinary service to the United States for decades."

"By any measure, he was one of the outstanding General officers of his generation, helping our military adapt to new challenges, and leading our men and women in uniform through a remarkable period of service in Iraq and Afghanistan, where he helped our nation put those wars on a path to a responsible end," Obama said. The White House has named Michael Morell, the agency's deputy director, to serve as acting director.

The decision abruptly ends the public-service career of one of the military's most vaunted leaders. He led the surge in Iraq, and was later tapped to lead U.S. and NATO forces in Afghanistan -- following two years at the helm of U.S. Central Command. In April 2011, Obama again tapped Petraeus to lead the CIA.

He leaves just three days after Obama was elected to a second term, and amid a challenging environment for the country's intelligence community -- which is dealing with not just rogue nations like Iran, but a changing landscape elsewhere as a result of the Arab Spring. It has been confirmed that the U.S. compound that was attacked in Libya housed CIA operatives as well as State Department staff.

The intelligence community subsequently came under scrutiny when some officials suggested the administration initially claimed the attack was "spontaneous" only because of the intelligence assessments at the time.

Director of National Intelligence James Clapper said Friday that Petraeus' resignation "represents the loss of one of our nation's most respected public servants."

Lawmakers on Capitol Hill expressed similar sentiments.

"General David Petraeus will stand in the ranks of America's greatest military heroes. His inspirational leadership and his genius were directly responsible -- after years of failure -- for the success of the surge in Iraq," Sen. John McCain, R-Ariz., said. "General Petraeus has devoted his life to serving the country he loves, and America is so much the better for it."

Fox News' Mike Levine contributed to this report.