You know, I don't recall exactly, but I remember it being very clear first season. Unless I've conflated something from somewhere else, which is possible.

Oh, wait, I know! It's actually metaknowledge--somewhere in the pre-production docs he's described as an academic psychologist recruited by the FBI specifically because of his knowledge of criminal psych. There's also an extensive description of his office, and how it looks like a college professor's office, not an FBI agent's. It may have also come up in an episode, but I have the knowledge from background reading.