David Petraeus resigned after his affair with Paula Broadwell was discovered. (Creative Commons)

CIA Director David Petraeus’ extramarital affair was first discovered through an email trace of his biographer and paramour Paula Broadwell, U.S. officials told The Associated Press on Saturday.

When harassing emails were sent by Broadwell, to another woman, the FBI began investigating Broadwell.

The probe led agents to her email account, which uncovered details into her relationship with the Petraeus.

It is not clear why the FBI waited until Election Day to tell U.S. Director of National Intelligence James Clapper, who oversees the CIA and other intelligence agencies, about its investigation involving Petraeus, Reuters reported.

The CIA has not revealed the identity of the woman who received the threatening e-mails by Broadwell, the Boston Globe reported.

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