A crime scene is really an accumulation of things that, when taken together and analyzed, tell the story of the crime. But in order to understand that story, you first need to find and collect all of the evidence. Here’s where careful observation comes in. Everyone’s first instinct is to ignore the big picture and head directly to the body or the central area where the crime occurred. But that kind of tunnel vision can lead you to misread the case. And if you just rush in, you risk contaminating or destroying crucial evidence. After all, the suspect had to get in and out of the scene somehow. Chances are, he or she left some evidence behind. If you pay attention, you can find that evidence.
Therefore, when you first arrive at a crime scene, you should always stop and carefully observe the entire scene before you do anything else. Once you’ve made your observations, then you can begin processing the scene.
From: Crime Scene Observation by Dick Warrington