(CNN) -- About four years ago, Paula Broadwell began her Ph.D. dissertation on Gen. David Petraeus' innovative leadership skills.
Some of the interviews were done via e-mail. Others were conducted as Broadwell ran with the physically fit four-star and his team along the Potomac River in Washington.
Broadwell decided to turn that research into a book and go to Afghanistan after Petraeus was tapped in June 2010 to replace Gen. Stanley McChrystal as the top commander in the country.
"We had a relationship before I went there as far as this dissertation was concerned, so it just took it to another level," Broadwell told CNN's Brooke Baldwin in February.
The relationship between Petraeus and his biographer has become the subject of speculation since he admitted to having an extramarital affair and resigned Friday as CIA director.
The affair came to light during an FBI investigation into a complaint that Broadwell was sending harassing e-mails to another woman close to Petraeus, 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.
CNN has not been able to reach Broadwell for comment. Her website was not operating Saturday.
Broadwell moved with her husband, Scott, to Charlotte, North Carolina, about three years ago, according to the Charlotte Observer.
Broadwell, who attended the U.S. Military Academy and served in the Army Reserves after active duty, quickly became a guest on TV news shows after her book, "All In: The Education of General David Petraeus," was published earlier this year.
In several appearances on CNN, Broadwell -- with her background in intelligence and counterterrorism -- was asked about U.S. policy in Afghanistan, the Syrian civil war and Iran's use of nuclear technology. She serves as a research associate at Harvard University's Center for Public Leadership.
Broadwell told Baldwin that she embedded with Petraeus' staff and troops in the field, sharing their hardships and risks. Broadwell spent months in Afghanistan interviewing Petraeus and others for her book, which she wrote with Vernon Loeb.
In January, Broadwell told Jon Stewart of "The Daily Show" that Petraeus is tenacious, has a will to win and wants to make a difference. "He loves serving. He loves to be in the arena," she said.
Just last week, Broadwell published on the Daily Beast a list of Petraeus' 12 rules for living.
"We all will make mistakes," read one rule. "The key is to recognize and admit them, to learn from them, and to take off the rear-view mirrors -- drive on and avoid making them again."
Broadwell met Petraeus in spring 2006 when he spoke at Harvard, where she was a graduate student, she wrote in the preface to "All In."
The author told the general about her research interests and he agreed to put her in touch with people studying the same issues. "I later discovered that he was famous for this type of mentoring and networking, especially with aspiring soldier-scholars," Broadwell wrote.
In the preface, Broadwell -- who earned varsity letters in cross-country and track -- said she wanted to see whether Petraeus could match her stride when they ran. "Instead, it became a test for me."
Broadwell's passion for physical fitness extended to her appearance earlier this year on "The Daily Show." In an effort to raise money for wounded veterans, she and Stewart engaged in a push-up contest.
Stewart and Scott Broadwell faced off against Paula Broadwell.
A game, but grimacing, Stewart made it to 38 push-ups.
Paula Broadwell, apparently ready to do more, stopped at 60.
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