Under some circumstances, the disclosure of an undercover agent's identity can be a federal crime, though that specific offense was never alleged in this case.

NPR: Reporter Miller Returns to Stand at Libby Trial

Exposing an undercover CIA agent's identity can be a felony.

NPR: Lewis 'Scooter' Libby Found Guilty of Lying

Now initially the investigation was thought to be about who disclosed the agent's identity, but now there's speculation that it may be a probe into whether any government officials lied to the grand jury or obstructed the investigation.

NPR: Second White House Official Named in Plame Leak