What does it take for the F.B.I to start digging into your private e-mail account?

According to several reports, the FBI began scrutinizing Paula Broadwell’s online activity after they began “tracing” fewer than 10 threatening emails sent to Jill Kelley, 37, of Tampa, Fla, from an anonymous e-mail account.

Kelley, a friend of the Petraeus family,  prompted the investigation after she complained to an F.B.I. agent – also a personal friend – about the emails. As reported by the New York Times, the emails accused Kelley of “inappropriate flirtatious behavior with Mr. Petraeus.”

The Washington Post reports that F.B.I officials described the e-mails as “threatening and harassing” but not specific enough to warrant criminal charges.

Before long, F.B.I. investigators discovered that the anonymous account belonged to Broadwell, as noted by the New York Times.

Because the sender’s account had been registered anonymously, investigators had to use forensic techniques — including a check of what other e-mail accounts had been accessed from the same computer address — to identify who was writing the e-mails.

Eventually they identified Ms. Broadwell as a prime suspect and obtained access to her regular e-mail account.

The rest, as they say, is history. After the FBI discovered sexually explicit emails from an account they traced to Petraeus, investigators concluded that he was involved in an affair with Broadwell.

The F.B.I didn’t interview Broadwell until after they discovered the explicit emails in her account. After she was confronted with the e-mails, Broadwell confessed the affair with Petraeus and turned over her personal computer.

The moral lessons are quite clear in this incident but the investigation timeline also serves as a warning to email users. It doesn’t take much for the F.B.I to start snooping in your private email accounts.

The Washington Post reports that “investigators thought they were dealing with a routine harassment case until some communications were traced to a private e-mail account belonging to Petraeus.”