Gen. Petraeus with Paula Brodwell, the woman who wrote his biography 'All In' and the one he had an affair with.

Jill Kelly (on right), is the woman that Paula Broadwell allegedly harassed via email.

A social planner's complaints about email stalking launched the months long criminal inquiry that led to a woman romantically linked to former Gen. David Petraeus and to his abrupt resignation Friday as Central Intelligence Agency chief.

The emails began arriving in Jill Kelley's inbox in May, U.S. officials familiar with the probe said. Ms. Kelley, who helped organize social events at MacDill Air Force Base in Tampa, Fla., told the Federal Bureau of Investigation about the emails, which she viewed as harassing, the U.S. officials said.

That FBI investigation into who sent the emails led over a period of months to Paula Broadwell, Mr. Petraeus's biographer, with whom he was having an extramarital affair, according to the U.S. officials.

FBI agents were pursuing what they thought was a potential cybercrime, or a breach of classified information.

Instead, the trail led to what officials said were sexually explicit emails between two lovers, from an account Mr. Petraeus used a pseudonym to establish, and to the destruction of Mr. Petraeus's painstakingly crafted image as a storied Army general.

Mr. Petraeus admitted to an affair in a letter to CIA employees announcing his resignation. Read more at The Wall Street Journal