(CNN) - Retired Army Gen. David Petraeus resigned from his post as CIA director 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.
-- Despite his resignation as CIA director last week, Petraeus will testify Friday morning before the House and Senate Intelligence committees about the September attack on the U.S. Consulate in Benghazi, Libya, that left Ambassador Chris Stevens and three other Americans dead.
-- Petraeus will tell lawmakers that his resignation was not linked to the attack, a concern raised by administration detractors, given the timing of his departure just before the start of the hearings.
-- A U.S. official told CNN that a CIA investigation of the "general conduct" of Petraeus includes whether he used agency resources in carrying out his affair with Broadwell.
The Petraeus affair
-- The FBI uncovered the affair between Petraeus, 60, and his biographer, Broadwell, 40, after Broadwell allegedly sent anonymous, harassing e-mails about Kelley -- first to Allen in May, and then to Kelley and her husband in June -- according to sources familiar with the case.
-- Allen told Kelley about the e-mail he received, and Kelley eventually told an FBI agent -- a friend of hers -- about the messages, sources say.
-- Kelley, 37, and her husband are friends of Petraeus and his family and friends of Allen. A Central Command spokesman said she is a volunteer with no official position at MacDill Air Force Base in Tampa, Florida, where the U.S. Central Command is headquartered.
-- The FBI determined that the messages came from Broadwell. One of the messages to Kelley was along the lines of "stay away from my guy," a U.S. official says.
-- During the investigation of the e-mails, other communications surfaced connecting Petraeus and Broadwell, who is 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." She 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 the summer of 2012, Petraeus' friend 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 over 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 the U.S. diplomatic compound in Benghazi.
The Allen allegations
-- The Defense Department's inspector general is investigating allegations that Allen sent inappropriate messages to Kelley, the department said Tuesday. The FBI told the department about the allegations 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 said, "If they got out, John Allen would be very embarrassed by them." The official added that there was no evidence of physical contact between the two.
Investigations and fallout
-- The FBI is reviewing the actions of Frederick Humphries, the agent who triggered