Principles of Crime Scene Investigation

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

A hand holding a soft brush dusting the surface of a car hood revealing fingerprints

The logic behind this principle allows investigators to link suspects to victims, to physical objects, and to scenes. Whatever evidence that can link a person to the scene is referred to as associative show. 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 law-breaking, reconstructive evidence allows investigators to gain an agreement of the actions that took identify at the scene. A broken window, a blood spatter blueprint, bullet paths and shoe prints tin can all reveal what really happened. This type of evidence answers the question, "How did it happen?"

To aid establish the linkage of people and things to a scene, the investigator may likewise collect known substances, called control samples. These can exist items such every bit fibers from carpeting at the scene, glass fragments, soil, vegetation and other trace evidence. Learn more about trace evidence ▸ If these are constitute on the suspect'southward clothing, in their vehicle or at their residence, information technology could provide circumstantial evidence linking the person to the scene.

For example, police are chosen to a residential neighborhood where a home invasion and burglary has just occurred. Investigators collect drinking glass fragments from a shattered cabinet door with a distinct blueprint etched into the glass. A tip leads investigators to a local man with a known history of break-in. Examination of the suspect'due south habiliment yields glass fragments with the same distinct pattern every bit 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 whatever person(s) who have admission to the scene who are not considered suspects.

Gauging the Value of Evidence

It is unique - If an detail is institute that helps narrow the possibilities of who might exist considered a doubtable, or the manner in which a criminal offence was committed, this evidence would be of use. Is an impression from a vehicle tire found in the clay at the scene? The tread impression can exist compared to others to determine the type of tire that was on the car. Is a shoe print left in the soil? The tread may help to identify the size and type of shoes it came from and the wear pattern could be used to friction match information technology to a specific pair. Learn more than about footwear impressions and tire tracks ▸

The hand of an apparently uncouncious person lying next to a firearm with a crime scene marker numbered five

It has a low probability of occurring past hazard - Considering the mathematical probabilities will help to determine the odds that a slice of physical evidence plant at the scene could appear only past coincidence. If Dna evidence found at the scene matches a suspect, the chances are exceedingly low that another person could have left this sample. But fifty-fifty evidence that has a much higher probability—for instance, a common type of shoeprint that is left in the soil—is even so valuable. When combined with other high probability evidence, these can help narrow the list of possible parties and build a compelling instance.

It is inconsistent - If an item is found that is out of place or inconsistent with the setting, or is out of character for the victim—for case if the victim was a non-smoker only a cigarette barrel is constitute at the scene—this could be an important bit of evidence.

A view from above of an orange box cutter and 2-arm forensic scale on the floor near a wall corner

Information technology is a concrete match - If trace show is found on the suspect or in his possession that matches something at the scene, this makes this item valuable as evidence. For instance, cleaved plastic parts or a cleaved fingernail that can be matched past fracture marks can demonstrate that two pieces were in one case a role of the same item.

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