The FBI discovered sexually explicit emails on David Petraeus' email account addressed to his mistress Paula Broadwell, including one that talked about "having sex under a desk," according to the Daily Mail.

Petraeus, 60, allegedly continued to storm Broadwell, 40, with thousands of messages even after she had ended their affair, the report says.

The sexually explicit emails were discovered by the FBI when it was investigating complaints that Broadwell or someone close to the former CIA Director had accessed his Gmail account, according to the Wall Street Journal.

CNN reports that the FBI's investigation began after a second woman who was close to Petraeus, complained that Broadwell was sending harassing e-mails to her, an U.S. official told the news channel.

Instead they uncovered explicit messages between Petraeus and Broadwell sent from his own Gmail account. It is being claimed that Broadwell used Petraeus' own Gmail account to send the harassing emails to the second woman.

NBC News is reporting that the FBI is currently investigating Broadwell for "improperly trying to access his e-mail and possibly gaining access to classified information."

But there are other versions as to what led the FBI to initiate an investigation on Petraeus. Investigators allegedly encountered evidence of the extramarital affair while investigating news leaks, according to Reuters.

Petraeus confessed to cheating on his wife of 37 years Holly with whom he has two children on Friday, as he resigned from his position.

In his resignation letter he explained his behaviour was "unnacceptable."

Broadwell co-wrote the biography "All In: The Education of General David Petraeus" which was released earlier this year. She is reportedly married to radiologist Scott Broadwell with whom she has two young sons, Lucien and Landon, and lives in Charlotte, North Carolina, UK's Daily Mail reports.

Broadwell was embedded with Petraeus in Afghanistan from July 2010 to July 2011 and was "afforded extensive access by General Petraeus, his mentors, his subordinates, and his longtime friends," according to a biography on her website.