(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 occasionally ran with the physically fit four-star, including one time with Petraeus 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.
Petraeus biographer's e-mails led FBI to affair, official says
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.
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During the investigation, other communications surfaced between Petraeus and Broadwell, a married mother of two, according to the official.
CIA Director David Petraeus stepped down Friday, November 9, 2012, citing an extramarital affair with his biographer, Paula Broadwell. Many questions surround the affair, including why it was necessary for Petraeus to resign and the future of his marriage to his wife, Holly. Here's a look at other U.S. sexual scandals that led to political stumbles and downfalls.
Photos: Public figures, private missteps
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."
How Petraeus changed the U.S. military
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.
Ticker: Rep wants Petraeus to testify
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Former actor and California Republican Gov. Arnold Schwarzenegger made headlines in 2011 when his longtime wife, journalist Maria Shriver of the Kennedy clan, filed for divorce after learning Schwarzenegger had fathered a son with the couple's housekeeper. Schwarzenegger recently began talking publicly about the affair, released an autobiography and made a return to acting. He has said he hopes to win Shriver back.
Former president Bill Clinton's denial of his affair with then-intern Monica Lewinsky jeopardized his seat in the Oval Office. News of the affair surfaced in 1998, and Clinton became the second president to be impeached by the U.S. House when he was brought up on charges of lying to a grand jury and trying to influence the testimony of others but wasn't removed from office. He is still married to Secretary of State Hillary Clinton.
Sen. David Vitter, R-Louisiana, issued an apology "for a very serious sin in my past" after his phone number showed up in the records of Pamela Martin and Associates, and escort service run by Deborah Jeane Palfrew, aka the "D.C. Madam" 
Former U.S. Rep. Anthony Weiner, D-New York, caused a stir in Washington in 2011 when he was caught using social media to communicate with at least six women other than his wife, Huma Abedin. Weiner left office in his seventh term in Congress. Shortly after his resignation, news broke the Abedin was pregnant with their first child. Today, the couple is still married, and Weiner is a stay-at-home dad to their son. Weiner rejoined Twitter earlier this month.
Detroit Mayor Kwame Kilpatrick apologized to his wife and the city after romantic messages, reported by the Detroit Free Press, indicated the Democrat was having an affair with his chief of staff. The chief of staff, Christine Beatty, resigned, but Kilpatrick, said he would not. 
John Edwards, the former North Carolina senator and Democratic presidential hopeful, saw his political career spin off track when he finally admitted in 2008 that he was unfaithful to his cancer-stricken wife, Elizabeth Edwards. Edwards at first denied the affair but ultimately came clean about fathering a child with his campaign videographer, Rielle Hunter. Prosecutors accused Edwards of illegally using hundreds of thousands of dollars in campaign contributions to keep his pregnant mistress under wraps, but he was granted a mistrial on May 31, 2012. Elizabeth Edwards died in 2010.
Former Sen. Larry Craig, R-Idaho, agreed to step down temporarily as the leading Republican on Senate committees after details came out about his 2007 arrest in an airport in Minneapolis, Minnesota. 
Former Rep. Mark Foley, R-Florida, resigned in September 2006 after he was accused of sending sexually explicit instant messages and e-mails to congressional pages. Florida authorities opened an investigation, as did the FBI. Foley later checked into a treatment facility for alcoholism.
Weeks after separating from his wife, Los Angeles Mayor Antonio Villaraigosa acknowledged he had been having an affair with a local television reporter. 
Days before the House voted to impeach President Clinton, Rep. Bob Livingston, R-Louisiana, admitted to cheating on his wife. 
When then-South Carolina Gov. Mark Sanford went MIA for nearly a week in June of 2009, his staff told the public he was out hiking the Appalachian Trail. But when the Republican was spotted at Atlanta's Hartsfield-Jackson Airport, he decided to come clean about the mysterious hiking trip and quite a bit more. Sanford admitted he was not hiking, but visiting his Argentinian mistress in Buenos Aires. Though his wife, Jenny, said she was open to reconciliation, Sanford was head-over-heels for paramour Maria Belen Chapur. The Sanfords divorced. He became engaged to Belen Chapur in August.
Eliot Spitzer earned a squeaky clean image as the attorney general of New York who took on Wall Street corruption from 1999 to 2006. From there, he moved to the governor's mansion in Albany in 2007. But the Democrat was stopped in his political tracks when his liaisons with high-paid prostitute Ashley Dupre surfaced, and he stepped down as governor in March 2008. He briefly went on to anchor and now hosts "Viewpoint'" on Current TV. He is still married to Silda Wall Spitzer.
In 2009, then Sen. John Ensign, R-Nevada, admitted having an affair with Cindy Hampton, a campaign aide and the wife of his former chief of staff, Doug Hampton. Investigators examined the former senator's efforts to assist the Hampton family by providing a payment of nearly $100,000, arranging lobbying work for Doug Hampton and possibly meeting with him on a lobbying matter in violation of Senate rules. 
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