As more revelations emerge about the circumstances surrounding the resignation of CIA Director David Petraeus, congress is demanding answers about why it was kept in the dark for so long.

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Republican Congressman Peter King told CNN he wanted answers about the timing.

"So it seems this has been going on for several months, and yet now it appears that they're saying the FBI did not realise until election day that Gen Petraeus was involved. It just doesn't add up," he said.

Senate intelligence committee chair Dianne Feinstein said the news had come like "a lightning bolt". She said she would be investigating why the FBI did not inform her committee, or even the White House, of when it first became aware of a potential problem.

President Obama himself was not told until two days after the presidential election, in a briefing from the National Intelligence Director James Clapper.

'Social liaison'

The FBI first began investigating allegations about harassing emails from Paula Broadwell, who was having the affair with the General, back in the summer.

It had been alerted by 37-year-old Jill Kelley, described as an unpaid "social liaison" to the MacDill air force base in Tampa, Florida. She and her husband were long-time friends of General Petraeus and his wife Jill, and had socialised with them on a number of occasions.

According to the Washington Post, she went to her local FBI office and asked them to track down the source of "threatening and harassing emails", a trail which eventually led to Mrs Broadwell.

Ms Kelley has declined to comment: a statement issued on Sunday by her lawyer appealed for privacy.

Shared access

Some of the messages contained passages which looked like they came from General Petraeus's own personal email account, not because he had been hacked, but because he apparently shared some access with Broadwell.

The Post, which talked to several people close to the General, says they were surprised he allowed her so much access, although no-one had any idea that there might have been any kind of personal relationship between the two.

In another twist, Foreign Policy reports that Broadwell gave a speech to the University of Denver this October which touched on the attack on the US diplomatic mission in Benghazi.

She claimed that the CIA had been holding a couple of Libyan militia members as prisoners in an annexe - claims which the agency has strongly denied, describing them as "uninformed and baseless".

She also claimed that the CIA had been asked for help: "They were requesting the, what's called the CINC's in extremis force - a group of Delta Force operators, our very, most talented guys we have in the military. They could have come and reinforced the consulate and the CIA annex that were under attack."

The CIA has insisted this was not the case, denying similar allegations already made by Fox News.

Primary witness

Officials have been at pains to stress that General Petraeus's departure has nothing to do with the events in Benghazi, nor with the closed hearings into the attack which are due to take place on the Hill later this week. Petraeus was supposed to be a primary witness.

The CIA's acting director, Michael Morell, will testify instead. There are already suggestions that he would be a good pick for the permanent post, given his solid reputation and respect among his intelligence and political peers.

President Obama now has a major reshuffle on his hands. A new defence secretary, a new secretary of state - and now a new head of the CIA. No chance of an easy transition, for his second term.

Felicity Spector writes about US politics for Channel 4 News