Crime scene
Crime scenes can be major sources of physical evidence that is used to associate or link suspects to scenes, victims to scenes, and suspects to victims. This is Locard's exchange principle. It is the basic tenet of why crime scenes should be investigated. Anything found at a crime scene can be physical evidence. In scientific crime scene investigation, the first activities at the crime scene are essential for the successful preservation of the physical evidence. The first responder and ultimately the crime scene investigator have the obligation to make the scene secure and ensure that any further activities at the scene do not change the evidence. The use of a multilevel security method would accomplish this task. The preliminary scene survey is the first on-scene activity. Precautions are taken for transient and conditional evidence during the survey.