David Petraeus believed he could keep his affair with Paula Broadwell secret even after he was interviewed by the FBI

Former CIA Director David Petraeus believed he could keep his affair with his biographer Paula Broadwell secret even after he was interviewed by the FBI, it emerged today.

After meeting David Petraeus on October 21 and also conducting two interviews with Paula Broadwell, the second on November 2nd, the FBI had concluded that no criminal charges would be brought.

But the FBI appears to have decided to inform the White House after a Florida-based agent informed a Republican member of Congress of the investigation.

This was the same agent who had triggered the inquiry when his friend, the socialite Jill Kelley, complained to him about anonymous harassing emails.

The fallout now threatens to engulf General John Allen of the US Marines, the NATO commander in Afghanistan. He is said to have exchanged between 20,000 and 30,000 pages of emails with Kelley since 2010.

To complicate matters further, the FBI agent had allegedly sent shirtless photos of himself to Jill Kelley before she reported the harassing emails to him and had been told not to interfere in the investigation.

The agent, depicted by government officials as a Right-winger, contacted Representative Dave Reichert and asked for a member of the House Republican leadership. On October 31st, Dave Reichert put him in contact with Representative Eric Cantor, House Majority Whip.

Eric Cantor informed the FBI of his conversation with the FBI agent, now himself under internal investigation, but did not make the matter public. Eric Cantor apparently knew of the FBI investigation into David Petraeus’ affair eight days before President Barack Obama.

The allegedly harassing emails were discovered to have been sent by Paula Broadwell. It is believed that John Allen received at least one email from the same email account used by Paula Broadwell to contact Jill Kelley.

John Allen is now under investigation by the Pentagon’s inspector general and his Senate confirmation hearings for the new post of NATO’s Supreme Allied Commander Europe, originally due on Thursday, have been put on hold.

A senior defence official told reporters that it was alleged there had been “inappropriate communications” between John Allen and Jill Kelley, Leon Panetta, Defence Secretary, said in a statement.

If John Allen follows David Petraeus into retirement it will leave Barack Obama with two gaping holes to fill at the heart of his national security team at a time when he is also expected to be replacing his secretaries of state, defence and treasury.

It is unclear exactly when FBI Director Robert Mueller and Attorney-General Eric Holder were informed although it has emerged that the investigation began as early as May.