May 2012 — Anonymous emails, ultimately traced to Broadwell, were sent to Gen. John Allen, the top U.S. commander in Afghanistan, and several other generals warning them to stay away from Kelley, the Florida socialite. The emails came from the pseudonym "Kelleypatrol" and included notes on Allen's plans to see Kelley in Washington the following week. Concerned about how anyone else would know about his personal plans, Allen forwarded the emails to Kelley to see whether she was playing a prank on them. Other generals also forward to Kelley copies of emails they received.
Early June 2012 — Kelley begins receiving additional emails directly from various anonymous email accounts claiming she's up to no good. One email mentions Petraeus and an upcoming social event in Washington.
June 2012 — Kelley discusses the emails with Humphries, the FBI agent, leading to an investigation. FBI agents are concerned that the sender of the emails is tracking the movements of Allen and Petraeus.
July 2012 — The affair between Petraeus and Broadwell ends, according to Petraeus' friend Boylan. The anonymous emails to Kelley stop. Because the FBI has designated the case a sensitive investigative matter involving national security issues, senior FBI and Justice Department officials are notified, including FBI Director Robert Mueller. Attorney General Eric Holder is notified around this time.
August 2012 — Humphries tells Kelley he's been removed from the case and complains that the FBI is moving too slowly. Meanwhile, another FBI agent involved in the investigations tells Kelley the FBI has traced the emails to Broadwell, whom Kelley has never met.
Late summer 2012 — Emails between Petraeus and Broadwell lead agents to believe the two are having an affair.
Late September — The FBI conducts the first of its two interviews with Broadwell. By this time, the FBI has long since concluded that none of Petraeus' computers had been hacked and that Petraeus was not involved in the cyber harassment of Kelley. After Broadwell was interviewed, agents search her computers and found substantial amounts of classified documents — a discovery that shifted the focus of the investigation.
October 2012 — FBI agents spend most of the month trying to resolve issues related to the classified documents that Broadwell has — working with the Pentagon to assess where she got the documents, their significance and whether she was authorized to possess them.
Oct. 26 — The FBI conducts its first and only interview of Petraeus. At the interview, Petraeus acknowledges the affair. Petraeus is questioned about the classified documents that are in Broadwell's possession, and he denies giving her any classified documents.
Oct. 27 — Humphries communicates with House Majority Leader Eric Cantor and says he's worried the FBI isn't aggressively pursuing a possible security breach. Word of Humphries concerns first reaches Rep. Dave Reichert, R-Wash., who arranges for Humphries to speak with Cantor.
Oct. 31 — Cantor's chief of staff calls the FBI chief of staff to inform him of the tip. Cantor is assured soon after that the FBI is on top of any possible vulnerability.
Friday, Nov. 2 — The FBI conducts what Attorney General Eric Holder later calls "a very critical interview" with Broadwell that convinces the Justice Department it knows enough about the case to inform the White House. In this second interview with Broadwell, the FBI is told how many classified documents she received and that none of them came from Petraeus.
Tuesday, Nov. 6 — As Americans cast their ballots on Election Day, the FBI informs Director of National Intelligence James Clapper of the investigation. Clapper calls Petraeus and urges him to resign.