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.