The concept known as "Locard's Exchange Principle" states that every time someone enters an environment, something is added to and removed from the scene. The principle is sometimes stated as "every contact leaves a trace," and applies to contact between individuals as well as between individuals and a physical environment. Law enforcement investigators should always assume that physical evidence is left behind at every scene.

The amount and nature of the physical evidence deposited will be largely dependent on the circumstances of the crime and may include:

• Trace evidence — fibers, hair, glass, paint chips.
• Biological evidence — blood, saliva, semen.
• Comparative evidence — latent fingerprints, tool marks.

Items of physical evidence are not always visible to the naked eye and may be easily overlooked. A methodical approach to the collection and preservation of evidence is essential. One exception is that if evidence integrity is at risk, it is important to make rapid decisions to prevent degradation or loss. Agency protocol dictates who collects this type of evidence.

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