The official said that 37-year-old Jill Kelley in Tampa, Fla., received the emails from Petraeus biographer Paula Broadwell that triggered an FBI investigation. Kelley was the State Department's liaison to the military's Joint Special Operations Command. The official was not authorized to discuss the case publicly and spoke on the condition of anonymity.
The nature of the relationship between Petraeus and Kelley wasn't exactly clear, but a statement released to ABC News by Kelley Sunday afternoon portrays the two as being longtime family friends.
"We and our family have been friends with Gen. Petraeus and his family for over five years. We respect his and his family's privacy and want the same for us and our three children," the statement read.
Another person who knows both Kelley and Petraeus confirmed their friendship, saying she saw him often.
Petraeus quit his post on Friday after admitting to having an extramarital affair with Broadwell, who had access to him when he was running the war in Afghanistan.
The FBI probe began several months ago when Kelly filed a complaint over the harassing emails. When the FBI traced the suspicious email to Broadwell's inbox, they also found emails indicating a romantic or sexual relationship between the married biographer and Petraeus, who has been married to his wife Holly for 37 years.
According to ABC News, the FBI uncovered no compromising of classified information or criminal activity, sources familiar with the investigation told ABC News, adding that all that was found was a lot of "human drama."
Sunday, Dianne Feinstein, the head of the Senate Intelligence Committee, said she wants to know why the FBI didn't tell her sooner that agents were investigating Petraeus' affair. The California Democrat told a news outlet that she first learned about the matter from media last week. She's since been briefed by the FBI, but still wants to know why the bureau didn't notify her sooner because of the national security implications.
Feinstein says she initially didn't want President Barack Obama to accept Petraeus' resignation, but realizes now he had no choice.
The news sent shockwaves across Washington. Petraeus was viewed as the military's most respected general of his generation. He is seen as a problem-solver and was entrusted with key roles by two presidents from different parties.
ABC News and The Associated Press contributed to this report.