The CIA has launched an internal inquiry following the resignation of its director David Petraeus, who confessed to an extramarital affair, a spokesman for the US intelligence agency has said.Petraeus, who was already the most celebrated US general of his generation before he took the helm at the CIA last year, resigned last week to pre-empt the revelation that he had had an affair with his biographer.CIA spokesman Preston Golson, said: "If there are lessons to be learned from this case, we'll use them to improve. "But we're not getting ahead of ourselves. An investigation is exploratory and doesn't presuppose any particular outcome."The scandal sent shockwaves through the Washington security establishment and entangled the US commander of allied forces in Afghanistan, General John Allen, whose career is on hold pending a Pentagon investigation.Petraeus has kept a low profile since his resignation, but he will be in the limelight once again on Friday when he heads to Capitol Hill to testify before two congressional committees over a deadly attack on a US mission in The 60-year-old retired general is not expected to face charges, but FBI agents seized a trove of documents from the home of his mistress, 40-year-old Paula Broadwell, and may act if she improperly received classified material.US Attorney General Eric Holder said on Thursday that the FBI had not found any "national security threat" in the course of its investigation.Petraeus's resignation also triggered speculation that its timing, three days after Barack Obama's re-election as US president, was political or was linked to the probe into the killing of four Americans at the Benghazi consulate.The general has yet to speak publicly, but he told a friend, CNN Headline News anchor Kyra Phillips, that the scandal had nothing to do with Benghazi and that he had never passed secret documents to his lover."He was very clear that he screwed up terribly, it was all his fault, even that he felt fortunate to have a wife who is far better than he deserves," Phillips said, after a series of calls to Petraeus."He has insisted to me that he has never passed classified information to Paula Broadwell," she added."He has said this has nothing to do with Benghazi and he wants to testify. He will testify."Petraeus will face both the House and Senate intelligence committees on Friday as they probe the September 11 attack on the consulate in Benghazi, in which Ambassador Chris Stevens and three more Americans died.