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Crime Scene and DNA Basics for Forensic Analysts

Types of Evidence

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Evidence can be divided into two categories:

  • Testimonial - statements or the spoken word from the victim(s) or witness(es).
  • Physical - also referred to as real evidence, consists of tangible articles such as hairs, fibers, latent fingerprints, and biological material.

The concept known as the "Locard's Exchange Principle" states that every time someone enters an environment, something is added to and removed from it. 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 are therefore taught to always assume that physical evidence is left behind at every scene. This will be generally true, and the amount and nature of the evidence created will be largely dependent on the circumstances of the crime.

Examples include:

  • Biological material - blood, semen or saliva
  • Fibers
  • Paint chips
  • Glass
  • Soil and vegetation
  • Accelerants
  • Fingerprints
  • Hair
  • Impression evidence – shoe prints, tire tracks or tool marks
  • Fracture patterns – glass fragments or adhesive tape pieces
  • Narcotics

Oftentimes, evidence tells a story and helps an investigator re-create the crime scene and establish the sequence of events. Physical evidence can corroborate statements from the victim(s), witness(es) and/or suspect(s). If analyzed and interpreted properly, physical evidence is more reliable than testimonial evidence; testimonial evidence is more subjective in nature. An individual's perception of events and memory of what happened can be incomplete or inaccurate. Physical evidence is objective and when documented, collected, and preserved properly may be the only way to reliably place or link someone with a crime scene. Physical evidence is therefore often referred to as the "silent witness."

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