The purpose of crime scene investigation is to help establish what happened (crime scene reconstruction) and to identify the responsible person. This is done by carefully documenting the conditions at a crime scene and recognizing all relevant physical evidence. The ability to recognize and properly collect physical evidence is oftentimes critical to both solving and prosecuting violent crimes. It is no exaggeration to say that in the majority of cases, the law enforcement officer who protects and searches a crime scene plays a critical role in determining whether physical evidence will be used in solving or prosecuting violent crimes.

Despite Hollywood's portrayal, crime scene investigation is a difficult and time consuming job. There is no substitute for a careful and thoughtful approach. An investigator must not leap to an immediate conclusion as to what happened based upon limited information but must generate several different theories of the crime, keeping the ones that are not eliminated by incoming information at the scene. Reasonable inferences about what happened are produced from the scene appearance and information from witnesses. These theories will help guide the investigator to document specific conditions and recognize valuable evidence.

Documenting crime scene conditions can include immediately recording transient details such as lighting (on/off), drapes (open/closed), weather, or furniture moved by medical teams. Certain evidence such as shoeprints or gunshot residue is fragile and if not collected immediately can easily be destroyed or lost. The scope of the investigation also extends to considerations of arguments which might be generated in this case (suicide/self defense) and documenting conditions which would support or refute these arguments.

In addition, it is important to be able to recognize what should be present at a scene but is not (victim's vehicle/wallet) and objects which appear to be out of place (ski mask) and might have been left by the assailant. It is also important to determine the full extent of a crime scene. A crime scene is not merely the immediate area where a body is located or where an assailant concentrated his activities but can also encompass a vehicle and access/escape routes.

Although there are common items which are frequently collected as evidence (fingerprints, shoeprints, or bloodstains), literally any object can be physical evidence. Anything which can be used to connect a victim to a suspect or a suspect to a victim or crime scene is relevant physical evidence. Using the "shopping list" approach (collecting all bloodstains, hairs, or shoeprints) will probably not result in recognizing the best evidence. For example, collecting bloodstains under a victim's body or shoeprints from emergency personnel will rarely answer important questions. Conversely, a single matchstick (not usually mentioned as physical evidence) recovered on the floor near a victim's body can be excellent physical evidence since it can be directly tied to a matchbook found in a suspect's pocket.

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