A second name emerged Sunday in the FBI investigation into CIA Director David Petraeus’ email correspondence that exposed an extramarital affair with a female biographer, then his resignation last week.

Sources tell Fox News the woman is 37-year-old Jill Kelley, a close friend of the Petraeus family.

Kelley alerted the FBI about the emails that appeared to be an attempt to blackmail Petraeus, which started the investigation, said the sources.

In addition, the threatening emails might not have come from biographer Paula Broadwell, as widely reported.

Kelley, a Tampa, Fla., resident who is married with three children, and sister Natalie are close friends of the Petraeus' and spent holidays together. And she was not having an affair with the retired, four-star Army general, sources close to the family tell Fox News.

Neither the State Department nor the military's Joint Special Operations Command could confirm Kelley worked for either, contrary to other published reports.

Kelley issued a statement through Smith and Company, a Washington communications and crisis management firm, asking for privacy. She says she respects Petraeus' privacy and wants the same for her own family.

Petraeus quit as CIA director last week after acknowledging an extramarital relationship with a woman -- later identified as Broadwell.

The relationship was an open secret with those who knew the Petraeus', sources also tell Fox News. The FBI probe at one point led to Broadwell's email account, which uncovered the relationship with Petraeus.

Petraeus’ resignation was announced Friday, but new details continue to emerge about his affair with Broadwell, a married mother and West Point graduate.

The first knowledge of the affair outside the FBI came from an agency whistle-blower who contacted a Capitol Hill Republican who told House Majority Leader Eric Cantor.

Rep. Dave Reichert, R-Wash., got a tip from a friend who knew the whistle-blower, as reported by The New York Times.

Sources tell Fox News that Reichert talked to the whistle-blower, then referred him to Cantor. The whistle-blower talked to the majority leader’s office, then to Cantor directly. The whistle-blower -- who purportedly was concerned about a possible national security breach -- was then put in touch with FBI Director Robert Mueller.

"Our office stands by the accuracy of the New York Times article as it pertains to Rep. Reichert,” the congressman’s office said Sunday.” We have no further comment about our involvement."

Cantor staffers said they didn't immediately tell the House Intelligence Committee or chamber leaders because they didn't know whether the tip was credible. So they turned it over to the FBI.

No security breaches appear to have occurred in the email exchange, but Capitol Hill lawmakers expressed shock and disappointment about the stunning revelations and said Petraeus made the correct choice in resigning.

“It was like a lightning bolt,” Democratic Sen. Dianne Feinstein, chairman of the Senate Select Committee on Intelligence, told “Fox News Sunday.” “I’m heartbroken. This is very, very hard. But I do think he did the right thing.”

Reports suggest the investigation started months ago and that Director of National Intelligence James Clapper was informed of the findings at 5 p.m. Election Day.

President Obama first learned Thursday when told by Petraeus, according to reporting so far.

Closed-door House and Senate intelligence committee hearings are scheduled Thursday on the incidents surrounding the fatal Libya attacks. Petraeus -- who reportedly conducted his own, on-the-ground investigation -- is no longer scheduled to testify.

There is no indication the scandal is related to the Libya situation.

However, Feinstein and Georgia Sen. Saxby Chambliss, the ranking Republican on the Senate intelligence committee, said Sunday that Petraeus could later be called to testify.

“I wouldn’t rule out Gen. Petraeus being called to testify,” Chambliss said on ABC’s “This Week.”

Broadwell told Fox News earlier this year when talking about the biography “All In: The Education of General David Petraeus” that she met Petraeus when she was a 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.”

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.

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.

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," he said. "The president graciously accepted my resignation."

Obama, in a written statement, said Petraeus provided "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 left 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.

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

Fox News’ Mike Levine, Chad Pergram and Jennifer Griffin contributed to this report.