Crime Scene Contamination and Evidence Preservation

Importance of Evidence Preservation

  • The intrinsic value of forensic evidence is critically and directly influenced by the actions taken, or not taken, immediately following the commission of a crime.
  • Risks to Unprotected Evidence: Evidence found at an unprotected crime scene is highly susceptible to degradation, diminishment, or complete disappearance over time. This natural deterioration and loss will occur unless the evidence is properly collected and meticulously preserved.

Understanding and Minimizing Contamination

  • Revisiting Locard's Principle of Exchange: It is crucial to recall Locard's principle, which postulates that every contact leaves a trace. In the context of crime scene processing, the very act of collecting evidence, regardless of how meticulously and carefully it is performed, will inherently result in a post-crime transfer of material. This means that investigators, by their presence and actions, unavoidably introduce or remove some trace elements from the scene.
  • Defining Contamination: Contamination is precisely defined as the introduction of any material to the crime scene that was not present originally at the time the crime was committed. This includes any foreign substance, human cells, fibers, or objects brought in by external sources.
  • Primary Cause of Contamination: A significant and notable finding, as cited by Warrington in 20142014, is that the majority of contamination occurring at crime scenes is actually caused by the very individuals tasked with investigating the scene. This highlights the critical need for stringent protocols to prevent investigator-induced contamination.
  • Strategies for Minimizing Contamination:
    • Crime Scene Security and Isolation:
      • To drastically minimize the potential for contamination of a crime scene and the valuable evidence contained within it, the immediate and paramount step is to effectively cordon off the entire area. This involves establishing a clear perimeter using tape or other barriers.
      • A fundamental rule is to keep all unnecessary personnel outside the designated scene perimeter. This explicitly includes police officers who are not directly assigned to processing the crime scene.
      • Impact of Unauthorized Presence: Law enforcement officers who are not assigned to the crime but venture into the scene, often out of curiosity, pose a substantial risk. Their unauthorized presence and movements can inadvertently obliterate existing clues, destroy fragile evidence, and even add extraneous trace evidence (e.g., fibers from their clothing, skin cells, shoe prints) that was not part of the original scene.
    • Personal Protective Equipment (PPE):
      • Another critically important measure to minimize contamination is to ensure that all personnel actively processing the crime scene wear appropriate protective clothing.
      • Examples of Essential PPE: This protective gear typically includes:
        • Tyvek®^{\tiny{\text{\textregistered}}} suits: These full-body suits are designed to cover the entire body, including the head, providing a barrier against the transfer of fibers, skin cells, and other particulates from the investigator to the scene, and vice-versa.
        • Disposable gloves: These are essential to prevent the transfer of skin cells, fingerprints, and other biological materials.
        • Shoe covers: Worn over footwear, these prevent the introduction of foreign particles from the soles of shoes and also prevent investigators from tracking out potential evidence.