It reads, in part:
1) Pristine crime scenes don't exist. In the show, each crime scene is manicured awaiting C.S.I. detectives arrival. Think about a 911 call. Shots fired in a house. Officers respond to the house and enter. They search for injured parties, possible suspects, and are making sure no one takes a shot at them. Once the sweep is performed and the area is secure, officers then look for evidence of a crime. You can imagine the condition of a crime scene once people have fought, someone was hurt, shots were fired, and now officers have walked through it.
Read more here.