The key principle underlying crime scene investigation is a concept that has become known as “Locard’s Exchange Principle”. It states that whenever someone enters or exits an environment, something physical is added to and removed from the scene. This principle is generally summed up by stating: “Every contact leaves a trace.”

The logic behind this principle allows investigators to link suspects to victims, to physical objects, and to scenes. Any evidence that can link a person to the scene is referred to as associative evidence. This may include items such as fingerprints, blood and bodily fluids, weapons, hair, fibers and the like. This type of evidence answers the question “Who did this”?

While associative evidence links people to the place of the crime, reconstructive evidence allows investigators to gain an understanding of the actions that took place at the scene. A broken window, a blood splatter pattern, bullet paths and shoe prints can all reveal that actually happened. This type of evidence answers the question, “How did it happen?”

To help establish the linkage of people and things to a scene, the investigator may also collect known substances, called control samples. These can be items such as fibers from carpeting at the scene, glass fragments, soil, vegetation and other trace evidence. If these are found on the suspect’s clothing, in their vehicle or at their residence, it could provide circumstantial evidence linking the person to the scene.

For example, police are called to a residential neighborhood where a home invasion and burglary has just occurred. Investigators collect glass fragments from a shattered cabinet door with a distinct pattern etched into the glass. A tip leads investigators to a local man with a known history of burglary. Examination of the suspect’s clothing yields glass fragments with the same distinct pattern as the smashed cabinet doors.

Eliminating people who could not be the perpetrator is also important. Control samples of fingerprints and DNA are often collected from any person(s) who have access to the scene who are not considered suspects.