Catherine: Lividity suggest that the victims died in the positions that they were discovered.
Riley: So, what does that mean? The killer is using some sort of gas chamber?
Grissom: I think so. He lures someone back to his places, slips them a sedative, then re-dresses and positions the bodies inside a sealed container, and waits for the carbon monoxide to go to work. Once the bodies are in full rigor, he's got about ten hours to place them and have them discovered.
Nick: Now that we know the recipe, let's find the cook.