NCIS: Season 6, Episode 15

6 - 15: Deliverance

While investigating the death of a Marine, the team finds Gibbs' Marine ID at the crime scene. It is revealed that he helped a Colombian woman 18 years ago and that her son (whose father Gibbs killed while on the mission there) tried to contact him about a major blackmailing, involving the theft of several crates of assault rifles from a Marine base.