The plot surrounding the shock resignation of CIA chief David Petraeus thickened yesterday with reports that his affair was exposed when the FBI investigated threatening e-mails from his lover to a mystery second woman.

Republicans, meanwhile, demanded more answers, pointing to the fact that as Petraeus had been days away from testifying about the September 11 attack on the US consulate in Benghazi, Libya, it smacked of some kind of conspiracy.

Petraeus, an American hero credited with turning the tide of the Iraq war, resigned on Friday after admitting an extramarital affair, sending shockwaves around Washington just days after President Barack Obama’s re-election.

It has emerged that his paramour was Paula Broadwell, a 40-year-old former Army major granted unprecedented access to the general as she co-authored a best-selling biography: “All In: The education of General David Petraeus.”

Newspaper reports yesterday revealed that the affair came to light as part of a criminal investigation launched when a second woman complained that she had received vicious e-mails from Broadwell. The “threatening and harassing” e-mails from Broadwell indicated she thought the other woman was a rival for the 60-year-old general’s affections, anonymous law enforcement officials told The Washington Post.

Both newspapers said the mystery woman was not Petraeus’s wife or a member of his family, but someone close to the general who does not work for the Central Intelligence Agency.

The recipient of the e-mails was so frightened, according to the Post, that several months ago she went to the FBI for protection and to help track down the sender.

The FBI soon uncovered Broadwell’s sexually explicit correspondence with Petraeus, leading to initial fears there may have been a national security breach with someone breaking into the CIA chief’s private email account.

Investigators interviewed Pet-raeus “two weeks ago,” law enforcement officials told the Post.