Retired Army Gen. David Petraeus resigned from his CIA director post last week after an FBI investigation revealed he had an extramarital affair, an investigation that also prompted questions about whether his paramour had inappropriate access to classified information.

The scandal also has sparked an investigation into whether Marine Corps Gen. John Allen, the top U.S. commander in Afghanistan, sent inappropriate messages to a different woman, leading President Obama to put Allen's nomination to become NATO's supreme allied chief on hold.

The FBI uncovered the Petraeus affair while it investigated e-mails that his paramour, Paula Broadwell, allegedly sent to a Petraeus family friend, Jill Kelley, according to a U.S. official. Kelley, meanwhile, is the woman to whom Allen allegedly sent inappropriate e-mails, according to the Defense Department.

Below is a summary of what we know about the situation.

New developments

-- Despite his resignation as CIA director last week, Petraeus is scheduled to testify Friday before the House Intelligence Committee about the September 11 attack that killed four Americans at a U.S. diplomatic compound in Benghazi, Libya, the committee said in a news release.

-- Petraeus has not been following media reports about his resignation, a former aide said Wednesday. "He wants to maintain a distance and focus on his family at this time," said retired Col. Peter Mansoor, who added that he had spoken earlier in the day with Petraeus. "He realizes it was a severe and morally reprehensible action, but he violated no laws."

-- A senior law enforcement official close to the Broadwell investigation said Wednesday night that it appeared unlikely she would be prosecuted for any unauthorized release of classified information. The official told CNN National Security Contributor Fran Townsend that investigators were reviewing materials taken Monday from Broadwell's home, but that the information in question did not appear to be substantial. The official stressed the decision whether to prosecute rests with the Justice Department.

The Petraeus affair

-- The FBI uncovered the affair between Petraeus, 60, and his biographer, Broadwell, 40, after Broadwell allegedly sent anonymous, harassing e-mails to Kelley in May, a U.S. official says. A senior official close to Allen says Allen received an anonymous e-mail about Kelley, and tipped Kelley off that someone was threatening her.

-- A source familiar with Kelley's version of events said the anonymous e-mails later traced to Broadwell -- which led to the discovery of Petraeus' affair -- began in June. It wasn't until two months later that the FBI told Kelley who had sent the e-mails, said the source, adding that Kelley does not know Broadwell and has never met her.

-- Kelley, 37, says she and her husband are friends of Petraeus and his family. Media reports have described her as a liaison at MacDill Air Force Base in Tampa, where the U.S. Central Command is headquartered. Petraeus and Allen were previously stationed there. A Central Command spokesman said she is a volunteer with no official position.

-- Investigators eventually traced the e-mails to Broadwell, a U.S. official said. The messages were along the lines of "stay away from my guy," but not explicitly threatening, according to a U.S. official.

-- During the investigation, other communications surfaced between Petraeus and Broadwell, a married mother of two living in North Carolina, an officer in the U.S. Army Reserve and a Ph.D. candidate in the Department of War Studies at King's College London.

-- Broadwell, a West Point graduate, had written a biography of Petraeus, published in January, called "All In: The Education of General David Petraeus." Having met Petraeus in 2006 when he spoke at Harvard, where she was a graduate student, she wrote the book after researching Petraeus for her Ph.D. dissertation on his leadership skills and visiting him and his team in Afghanistan, where he became top U.S. commander in 2010.

-- Petraeus and Broadwell began their affair in fall 2011, a few months after he returned to the United States, retired from the Army and took over at the CIA, according to a Petraeus friend. They ended it in summer 2012, Petraeus' friend said.

-- FBI investigators, following up on the anonymous e-mails to Kelley, discovered on Broadwell's computer e-mails that turned out to be from Petraeus, a U.S. official said.

-- On Election Day, November 6, Petraeus told Director of National Intelligence James Clapper about the affair, according to a senior U.S. intelligence official. Clapper advised Petraeus to resign, the official said.

-- On November 9, Petraeus quit the CIA, admitting to the affair. The House and Senate intelligence committees were informed of the FBI investigation the same day.

-- Petraeus' resignation came just days before he was scheduled to testify before the Senate Intelligence Committee about the September 11 attack that killed four Americans at a U.S. diplomatic compound in Benghazi, Libya.

The Allen allegations

-- The Defense Department's inspector general is investigating allegations that Allen sent inappropriate messages to Kelley, the department said on Tuesday. The FBI told the department about the allegations on Sunday.

-- Allen has denied wrongdoing, a senior defense official said. Sources familiar with Kelley have said the relationship between the two was not sexual. Authorities are looking at the e-mails.

-- "There is no affair" between Allen and Kelley, a senior official close to Kelley said. "She is a bored rich socialite involved with every single senior commander at CENTCOM, because she worked as an honorary ambassador."

-- A U.S. official familiar with the e-mails Allen sent to Kelley described them as warranting the investigation. "If they got out, John Allen would be very embarrassed by them," said the official, who added that there was no evidence of physical contact between the two. A second U.S. official, who has had the e-mails described to him, characterized their content as "sexy," but could not say whether they "crossed the line."