CSI: Crime Scene Investigation Season 4 Episode 15

Two masked men show up at a house and kill the husband and wife execution-style with two bullets at the back of the head. The man's body is then dragged from the fairly secluded driveway towards the street, a fact that greatly puzzles Gil Grissom. The couple lived at a very good and secured community with only two entrances, the main gate to the whole complex, and a personal back gate.