NCIS: Season 3, Episode 20

3 - 20: Untouchable

Due to suspicions of a mole working inside the Pentagon, Tony and Ziva are charged with interviewing members of the Pentagon's cryptography department. When one of their interviewees is found dead in her home, the team investigates her apparent suicide. While the cryptography department is immediately locked down, Abby uses forensic science to prove that a second person was in the room when the gun was fired.