CSM Marvin L. Hill Paula Broadwell with Gen. David Petraeus.

The biographer for resigning CIA Director David Petraeus is under FBI investigation for improperly trying to access his email and possibly gaining access to classified information, law enforcement officials told NBC News on Friday.

Paula Broadwell is the author of Petraeus' biography, "All In." She had extensive access to Petraeus in Afghanistan and has given numerous television interviews speaking about him.

Petraeus resigned Friday, citing an extramarital affair. The woman involved was not identified.

Law enforcement and multiple U.S. officials told NBC News that emails between Petraeus and Broadwell were indicative of an extra-marital affair.

Earlier, law enforcement officials said they do not believe the FBI investigation will result in any criminal charges. They also stressed that Petraeus is not under investigation.

The CIA declined all comment on the case. Broadwell could not be reached for comment.

Broadwell's Twitter account describes her as a national security analyst and Army veteran. A biography on her website, which went offline Friday evening, said she is married to a radiologist and has two children, both boys. The family lives in Charlotte, N.C. The biography said she is a West Point graduate and a research associate at Harvard University's Center for Public Leadership and a doctoral candidate in the Department of War Studies at King's College London.

Richard Engel is NBC News' chief foreign correspondent. NBC News' Robert Windrem, Kristen Welker, Michael Isikoff and Pete Williams contributed to this report.

NBC's Richard Engel and the Washington Post's David Ignatius discuss the breaking news of CIA director David Petraeus' resignation.

Author Paula Broadwell joins Morning Joe to discuss her new book on Gen. David Petraeus, bringing attention to troops who suffer from PTSD, and the relationship between Gen. Petraeus and Gen. Stanley McChrystal.

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