The affair that brought an end to David Petraeus' tenure as CIA director came to light during an FBI investigation into a complaint that his biographer Paula Broadwell was sending harassing e-mails to another woman close to him, a U.S. official said Saturday.

During the investigation, other communications surfaced between Petraeus and Broadwell, a married mother of two, according to the official.

The official did not identify the woman who made the initial complaint and did not know the nature of her relationship with Petraeus.

The FBI interviewed Petraeus in the course of its inquiry, said the official, who stressed that the CIA director was never the target of the investigation and his communications were never compromised. The official did not know whether Broadwell was interviewed.

The official did not have an exact timeframe for the investigation and could not say if it is still ongoing.

CNN has not been able to reach Broadwell for comment.

The Obama administration first learned of the affair in a phone call from the FBI to Director of National Security James Clapper at 5 p.m. on election night, according to a senior U.S. intelligence official.

On Saturday, questions arose about why congressional leaders were not informed of the investigation immediately.

According to a congressional aide familiar with the matter, the House and Senate intelligence committees weren't informed that there was an FBI investigation into Petraeus until Friday.

"The committees are required to be kept informed of significant intelligence activities," the aide said Saturday. "If there was an official investigation that was looking, at least in part, at information that was compromising the CIA director, then I think there's a solid argument to say that the committee leadership should have been notified to at least some level of detail."

The resignation also comes days before Petraeus was slated to testify before the Senate Intelligence Committee about the September 11 attack on the U.S. Consulate in Benghazi, Libya. The attack, in which four Americans were killed, became a point of contention during the presidential campaign.

Some have even suggested that the timing of Petraeus' stepping down is suspect, given the expected grilling in Congress. Acting CIA Director Michael Morell will testify instead.

"Director Petraeus' frank and forthright letter of resignation stands on its own," said a senior U.S. official, who spoke on the condition of anonymity because of the sensitivity of the topic. "Any suggestion that his departure has anything to do with criticism about Benghazi is completely baseless."

Homeland Security Committee Chairman Rep. Peter King, R-New York, who is also a member of the House Permanent Select Committee on Intelligence, insisted that Petraeus should not back out of plans to testify.

King, a vocal critic of the Obama administration's handling of the Benghazi attack, said on CNN's "Erin Burnett OutFront" that Petraeus is "an absolutely essential witness, maybe more than anybody else."

"David Petraeus' testifying has nothing to do with whether or not he's still the CIA director, and I don't see how the CIA can say he's not going to testify," King said.

"I think his testimony is certainly valuable, it's certainly necessary," King continued. "He was at the center of this, and he has answers that only he has."

If Petraeus does not testify as originally scheduled on Thursday, King said, "It should be very soon after that."

Broadwell spent a year with Petraeus in Afghanistan interviewing him for the book she co-wrote, "All In: The Education of General David Petraeus."

Petraeus' departure Friday appeared to be an abrupt end to a spectacularly successful career in public service.