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 prompted an investigation into whether Marine Corps Gen. John Allen, the top U.S. commander in Afghanistan, sent inappropriate messages to a different woman, prompting 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 Petraeus affair, the Allen allegations, and the investigations and fallout.

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 have been friends of Petraeus and his family for five years. Widepsread 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 at the base. 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." She wrote the book after researching, visiting and interviewing Petraeus for her Ph.D. dissertation on his leadership skills.

-- Broadwell met then-Gen. Petraeus in Spring 2006 when he spoke at Harvard, where she was a graduate student, according to the preface of her book. She told the general about her research interests.

-- In 2008, she began her dissertation on him, interviewing him by e-mail and during occasional jogs with him, including one they took with his team along the Potomac River in Washington.

-- In 2010, Petraeus hwas tapped to replace Gen. Stanley McChrystal as the top commander in Afghanistan. Broadwell decided to turn her research into a book and follow Petraeus to that country. She spent months in Afghanistan interviewing Petraeus and others for her book, which she wrote with Vernon Loeb.

-- In August 2011, Petraeus retired from the U.S. Army and left Afghanistan. The next month, Petraeus was sworn in as CIA director and returned to the Washington area.

-- Petraeus and Broadwell began their affair in fall 2011, about two months after he took over at the CIA, according to a Petraeus friend.

-- Broadwell and Petraeus ended their affair in summer 2012, a decision reached mutually, and the two last talked about a month ago, Petraeus' friend said.

-- FBI investigators, following up on the anonymous e-mails to Kelley, interviewed Broadwell twice this fall, a U.S. official said. They also gained access to her computer and discovered e-mails that turned out to be from Petraeus, who also was interviewed once, a U.S. official said. A source told CNN national security contributor Fran Townsend that Paula Broadwell was acting as Petraeus' archivist, and that the FBI went to Broadwell's house November 12 to look for any documents she might have. It was not clear whether any of the material there was classified, the source 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 then 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, prompting outrage from House and Senate members.

-- 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 into some 20,000 to 30,000 pages of documents, a defense official told CNN. It is not clear how many of those include potentially inappropriate communications.