Jerome Sax was a contract killer responsible for the death of NCIS Special Agent Jack Patterson. He also infiltrated the NCIS building pretending to be an old co-worker of Patterson in order to get information on the case they were working on.
When Gibbs and Tony traveled to the desert to rescue the artist, Dina Risi that gave Patterson the evidence, Sax arrived with another man on an helicopter and attempted to kill the group with a machine gun.
Gibbs grabbed a gun and using his sniper training, shot the pilot which resulted in the helicopter going out of control and crashing into a mountain with the impact of the crash killing both the pilot and Sax in the process.
Agent Timothy McGee later revealed that Sax had been married with two kids and that Sax's wife had been oblivious to her husband being a contract killer.