WASHINGTON  — David Petraeus, the retired four-star general renowned for taking charge of the military campaigns in Iraq and then Afghanistan, abruptly resigned Friday as director of the CIA, admitting to an extramarital affair.

The affair was discovered during an FBI investigation, according to officials briefed on the developments. They spoke on condition of anonymity because they were not authorized to publicly discuss the matter. It was unclear what the FBI was investigating or when it became aware of the affair.

Petraeus’ resignation shocked Washington’s intelligence and political communities. It was a sudden end to the public career of the best-known general of the post 9/11 wars, a man sometimes mentioned as a potential Republican presidential candidate. His service was effusively praised Friday in statements from lawmakers of both parties.

Petraeus, who turned 60 on Wednesday, told CIA employees in a statement that he had met with President Barack Obama at the White House on Thursday and asked to be allowed to resign. On Friday, the president accepted.