"...A decision was made somewhere not to brief us, which is atypical," Feinstein said. "This is certainly an operationally sensitive matter. But we weren't briefed. I don't know who made that decision."
The FBI investigation began when Kelley went to FBI officials to complain that Broadwell was sending harassing e-mails to her, a U.S. official told CNN. Kelley received the worrisome e-mails in May, an official said, describing the messages as along the lines of "stay away from my guy," but not explicitly threatening.
According to a source with knowledge of the e-mails, the messages accused Kelley of untoward behavior with some generals at MacDill Air Force Base in Tampa, Florida where Kelley did volunteer work.
The e-mails detailed the "comings and goings of the generals and Ms. Kelley," said the source, who declined to speak on the record because of sensitivity of the investigation.
Among those believed to be referenced in the e-mails was Petreaus. Because parts of Petreaus' schedule were not public, the e-mails raised questions about whether the sender of the e-mails had access to his private schedule or other sensitive information.
The content of the e-mails was first reported by NBC News.
At one point, Petraeus told Broadwell to stop sending the e-mails, a U.S. official said. It was not clear whether his request was made during or after his affair with Broadwell.
During the investigation, other communications surfaced between Petraeus and Broadwell, a married mother of two, according to a U.S. official.
Petraeus used a personal account to e-mail Broadwell, not his CIA account, a U.S. official said.
The FBI interviewed Petraeus, said the official, who stressed that the CIA director was never the target of the investigation and his communications were never compromised.
Broadwell was interviewed twice by the FBI, a U.S. official said.
The official said the investigation is not officially closed, but it appeared there will be no charges.
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 the situation 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."
But former FBI assistant director Tom Fuentes told CNN on Monday that if, as the investigation progresses, the FBI is not "uncovering criminal activity" or a "breach of security" then "there really isn't a need" to notify members of Congress.
The FBI has "very strict protocols" about who should be notified in this type of investigation, Fuentes said.
House Majority Leader Eric Cantor, R-Virginia, knew in October about Petraeus' involvement in an extramarital affair, a spokesman for the congressman told CNN on Sunday.
Doug Heye said Cantor was tipped to the information by an FBI employee. The congressman had a conversation with the official, described as a whistle-blower, about the affair and national security concerns involved in the matter, he said.
Broadwell and Petraeus first met in 2006 at Harvard University's Kennedy School of Government, where she was a student, Broadwell wrote in the preface of the biography she co-authored on Petraeus.
She told him about her interest in studying military leadership, and he offered his help.
"I later discovered that he was famous for this type of mentoring and networking, especially with aspiring soldier-scholars," Broadwell wrote.
She traveled to Afghanistan, where she interviewed Petraeus repeatedly, sometimes on long runs that likely increased the general's respect for her.
"She probably kicked his butt," Marks said. "And it was probably the first time that had ever happened to him, so he let his guard down. He brought her in."
Such runs were a common way for Petraeus to conduct business, an adviser who worked on and off in Afghanistan with Petraeus and Broadwell told CNN. Still, some staffers were jealous of the access she had to him and the lengthy interactions they had, the adviser said.
Out of those conversations and months of other research came the best-selling book "All In: The Education of General David Petraeus."
In promoting the book, and defending it against critics who said it was too sympathetic, she frequently spoke of her unprecedented access to the general and glowingly of his character.
"I'm not a spokesperson for him, and if showing a role model to other people in the world or other readers is a repugnant thing, then I'm sorry, but I think the values that he upholds and tries to instill in his organizations are valuable and worth pointing out," she told CNN in February.
The actual affair began about two months after Petraeus took over at the CIA in September 2011, according to one Petraeus friend.
It ended about four months ago, and the two last talked about a month ago, the friend said.
The decision to end the relationship was mutual, the friend said.
Another of the former general's friends said Petraeus felt isolated after leaving the camaraderie of the military, and it made him more vulnerable.
"I think leaving the Army, the emotions, and the psychological effect impacted on him more than he thought it would," the friend said.
Broadwell, with her background in military and intelligence issues, was someone he could talk to, the friend said.
"He enjoyed her company," the friend said. "She was an attractive gal and they had things in common."
According to her biography at the University of Denver, Broadwell graduated with honors from the U.S. Military Academy and has had "assignments with the U.S. intelligence community, U.S. Special Operations Command and an FBI Joint Terrorism Task Force."
She is a now a doctoral student at King's College in London, where her webpage indicates she is interested in studying military leadership, organizational and management theories and U.S. foreign, defense and intelligence policies.
CNN has not been able to contact Broadwell for comment. Her house in Charlotte did not appear occupied Monday.
Former Petraeus spokesman Steve Boylan said the retired general is devastated by the incident, and focused on his family.
"Furious would be an understatement," to describe Petraeus' wife, he said.