There's no such thing as a truly 'anonymous' email account, and no matter how much you try to encrypt the contents of the email you are sending, little fragments of data are attached by email servers and messaging companies. It's how email works and it's entirely unavoidable...which first led the FBI on a path that led up to the very door of Petraeus' office door in Langley, Virginia.
In this case, however, the Foreign Intelligence Services Act (FISA) would not apply. Even the Patriot Act would not necessarily apply in this case, even though it does allow the FBI and other authorized agencies to search email. However, in this case, above all else, the Stored Communications Act does apply -- part of the Electronic Communications Privacy Act.
The act allows for any electronic data to be read if it has been stored for less than 180 days. In this case, the law was specifically designed -- albeit quite some time before email became a mainstream communications medium -- to allow server- or computer-stored data to be accessed by law enforcement.
However, a court order must be issued after the 180 days, and in this case it was...
Once it knew Ms. Broadwell was the sender of the threatening messages, the FBI got a warrant that gave it covert access to the anonymous email account. And that's how they do it. (more)
