TIL there was a FBI mole that was given a task to unmask the FBI mole (himself)

Robert Hassen
http://en.wikipedia.org/wiki/Robert_Hanssen

Also, a cool movie about his arrest: Breach (2007)

One thing I find fascinating is that it was not clear about his motivation to betray FBI. The movie seems to suggest he did not get anywhere with his career in FBI, and just wanted the rush to be someone important. Also, he has a double life with regard to his marriage as well. He is associated with strippers and frequently invites others to observe his sex life, which further suggests he just wanted to feel important and special.