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.

The Petraeus affair

- The FBI uncovered an affair between Petraeus, 60, and Petraeus biographer Paula 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 it was Allen who received an anonymous e-mail about Kelley, and tipped Kelley off that someone was threatening 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, Florida, where the U.S. Central Command is headquartered. Both 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 an 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 Gen. John Allen, the top U.S. commander in Afghanistan, sent inappropriate messages to Kelley, the department said November 13. The FBI told the department about the allegations on November 11.

- The potentially inappropriate messages were "flirtatious" in nature, a defense official who has been authorized to speak on the matter told CNN.

- Allen has denied wrongdoing, a senior defense official said. 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."

Investigations and fallout

- Broadwell’s affair with Petraeus has raised questions about whether the affair gave her access to national security information that she shouldn’t have. The allegedly harassing e-mails that the FBI allegedly tracked to Broadwell detailed the "comings and goings of the generals and Ms. Kelley," according to a source. Among those believed to be referenced in the e-mails was Petraeus. Because parts of Petreaus' schedule were not public, the e-mails raised questions about whether the sender had access to sensitive information.

- In a speech at the University of Denver at the end of October, Broadwell suggested the attack on the U.S. compound in Benghazi took place because the United States was housing Libyan prisoners there - a theory, she noted, that had not been vetted yet. The claim has since been discounted by administration officials. Broadwell's source for that previously unpublished information remains unclear, and there's no evidence that it came from Petraeus.

- A senior U.S. intelligence official said Broadwell did not have a security clearance from the CIA. Another official said Broadwell, who is an officer in the Army Reserve, did have some kind of security clearance and that there are no issues with Broadwell having unauthorized access to classified information.

- Sen. Dianne Feinstein, D-California and chair of the Senate Intelligence Committee, said she would investigate why the FBI did not notify congressional oversight committees about its investigation into Petraeus after the bureau determined he was having a secret extramarital affair.

- Because Petraeus resigned as CIA director, he won’t testify before Congress about the Benghazi attack as scheduled. Some Republicans have criticized the administration's response to the Benghazi attack and have speculated that the timing of Petraeus' departure was linked to the congressional inquiry.

- Feinstein said that she still hopes to bring Petraeus to testify about the Benghazi attack on November 16.

- While the nature of the relationship between Allen and Kelley, if any, is unclear, evidence of an affair could subject the general to military prosecution. Adultery is a violation of military law.

- President Barack Obama has put Allen’s nomination to become NATO's supreme allied commander on hold pending the outcome of the investigation, White House spokesman Jay Carney said on November 13.