The nation's chief intelligence officer, "narc'd out"

This weekend, more details about the Petraeus affair emerged. FBI investigators -- in a twist of irony fit for either a crime drama or a soap opera -- discovered the CIA director's infidelity by accident: An acquaintance of the Petraeuses, a social liaison to MacDill Air Force Base in Tampa, where the military's Central Command and Special Operations Command are located, seems to have unknowingly triggered the series of events that would lead to CIA director's resignation. And, even more ironically, it was the particulars of the ubiquitous system Petraeus used to communicate with Paula Broadwell -- email (in this case, Gmail) -- that would lead investigators to discover information that they weren't actually seeking to find.

Sometime in May, The New York Times reports, Broadwell apparently began sending emails to Jill Kelley, the Petraeus acquaintance (her precise connection to the family isn't yet fully clear) -- and those emails were "harassing," according to Kelley. The messages were apparently sent from an anonymous (or, at least, pseudonymous) account. Kelley reported those emails to the FBI, which launched an investigation -- not into Petraeus, but into the harassing emails. 

From there, the dominoes began to fall. And they were helped along by the rich data that email providers include in every message they send and deliver -- even on behalf of its pseudonymous users. Using the "metadata footprints left by the emails," the Wall Street Journal reports, "FBI agents were able to determine what locations they were sent from. They matched the places, including hotels, where Ms. Broadwell was during the times the emails were sent." From there, "FBI agents and federal prosecutors used the information as probable cause to seek a warrant to monitor Ms. Broadwell's email accounts."

They received that warrant. And then domino ... domino ... domino.

They learned that Ms. Broadwell and Mr. Petraeus had set up private Gmail accounts to use for their communications, which included explicit details of a sexual nature, according to U.S. officials. But because Mr. Petraeus used a pseudonym, agents doing the monitoring didn't immediately uncover that he was the one communicating with Ms. Broadwell.

By late summer, after the monitoring of Ms. Broadwell's emails uncovered the link to Mr. Petraeus, prosecutors and agents alerted senior officials at FBI and the Justice Department, including Mr. Holder, U.S. officials say. The investigators never monitored Mr. Petraeus's email accounts, the officials say.

Of course, though, they didn't need to. Email, information-wise, says as much about its sender as its receiver. The messages' metadata had told investigators what they needed to know, even if they weren't looking to know it in the first place. 

Update: I've updated this post to reflect the fact that the "metadata footprints left by the emails" referred to by the Journal may or may not have been Gmail footprints. Though part of the FBI probe, it's been established, concerned an inquiry into Petraeus's Gmail account, it's ostensible that there could have been several other email accounts involved -- Gmail or other kinds.