Petraeus's announcement of resignation last Friday after confessing to an extramarital relationship shocked Washington.
Media reports over the weekend said that the FBI began investigating the case after Kelly, a friend of the Petraeus family, complained to law enforcement about receiving anonymous and threatening emails. The FBI traced the emails to Paula Broadwell, Petraeus's biographer, and then discovered that Broadwell was Petraeus' girlfriend.
Petraeus, a retired four-star general who served more than 37 years in the U.S. Army, took the office as the director of CIA in September, 2011. He had overseen all coalition forces in Iraq and led the military campaign in Afghanistan.
U.S. lawmakers on both sides have criticized the FBI for not notifying the Congressional intelligence and security panels of the probe into Petraeus's scandal.
Democratic Senator Dianne Feinstein, chairwoman of the Senate Intelligence Committee, said on Monday that her concern about the case had escalated over the last few days as more details popped up.
The panel, she said, will investigate why the FBI failed to alert the panel immediately about Petraeus's scandal.
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