NCIS (2003) : 3x16

When an ambulance explodes while carrying the remains of a Marine's body that was killed in a car crash, the NCIS team is called in to investigate what actually happened. The investigation leads them to some sudden discoveries -- the DNA of the victim found in the ambulance doesn't match the Marine's DNA; the ambulance was set up to explode; and the family of the dead Marine is hiding a very important secret.