In-Depth Notes on Trace Evidence

INTRODUCTION TO TRACE EVIDENCE

  • Overview of trace evidence and its importance in forensic science.

DEFINING TRACE EVIDENCE

  • Two Notions of Trace Evidence:
    1. Related to Size:
    • Small amounts of evidentiary material, often not visible to the naked eye.
    1. Something Left Behind:
    • Example: "He vanished without a trace!" (implying a lack of physical evidence).
    • Closely related to Transfer Evidence:
      • Marks made and materials transferred during criminal activities.
      • Involves small, transferable materials exchanged between people, places, or objects during a crime.

INVESTIGATIVE AID

  • Purpose of Trace Evidence:
    • Can provide information about the origin/date of manufactured materials and geographic provenance.
    • Helps in establishing associations and links among individuals, scenes, and items.
    • Aids in the reconstruction of the manner/place of contact during a crime (e.g., hit-and-run cases).

THE BASIS OF EVIDENCE: TRANSFER AND PERSISTENCE

  • According to Locard’s Exchange Principle:
    • Material is always exchanged when contact occurs between two items.
  • Conditions Affecting Transfer Evidence:
    • Pressure, nature and number of contacts, type of material, degree of shedding, and area of contact.

TYPES OF TRANSFER EVIDENCE

  • Direct Transfer:
    • Evidence transferred directly from a source to a location without intermediaries.
  • Indirect Transfer:
    • Involves one or more intermediate objects in the transfer process.
  • Types of Transfers:
    • Primary Transfer: From a source to another surface directly.
    • Secondary Transfer: Previously transferred evidence sent to another surface.
    • Tertiary Transfer: Evidence transferred three times in total.
    • Non-contact Transfer: Evidence becomes airborne and settles on a surface (e.g., broken glass).
    • Pattern Transfer: Evidence leaving a pattern which holds greater evidential value (e.g., shoe prints).

FACTORS AFFECTING TRANSFER

  • Key Influencing Factors:
    • Material type, shedding degree, receiving surface's nature, contact nature, frequency, and contact area size.

PERSISTENCE OF EVIDENCE

  • Trace evidence will persist until it:
    • Undergoes further transfers, degrades, or is collected.
  • Factors Affecting Persistence:
    • Surface nature, transferred material, surrounding environment/activities, and time before collection.

EVIDENCE RECOVERY AT THE SCENE

  • Trace evidence is often invisible and may be overlooked at crime scenes.
  • Critical to approach scenes scientifically to avoid altering evidence.
  • Individuals like first responders and photographers can inadvertently contaminate or change crime scenes.

COLLECTION OF TRACE EVIDENCE

  • Collection Methods:
    • Collecting whole items (bedding, clothing).
    • Using gloves or tweezers for small evidence (glass, fibers).
    • Vacuum sweeping and tape lifting for invisible materials.
    • Ensuring proper transfer and chain of custody during handling.

KNOWN AND QUESTIONED EVIDENCE

  • Questioned Evidence:
    • Unknown origin (e.g., broken glass with blood).
  • Known Evidence:
    • Known source (e.g., glass from a broken window, fibers from clothing).

CONTROLS IN EVIDENCE ANALYSIS

  • Control Evidence:
    • Known source used for comparative analysis.
    • Caution: ensures tests are valid to avoid false positives/negatives.

TRACE EVIDENCE ANALYSIS

  • Analytical Methods:
    • Group items by origin (victim, scene, suspect).
    • Avoid mixing known and questioned evidence.
    • Utilize macroscopy, microscopy, and photographs for analysis.

GUIDELINES ON TRACE EVIDENCE HANDLING

  • SWGMAT Trace Recovery Guidelines:
    • Procedures for documentation, detection, collection, and preservation of evidence.
    • Emphasize significance of proper handling to determine associations and its evidential value.

EXAMPLES OF GUIDELINES

  • Ensure chain of custody is followed, avoid contamination, and document potentially compromised evidence.
  • Use the least intrusive methods for collection and preserve significant evidence properly.
  • Types of Preservation:
    • Paper packets, Petri dishes for secure evidence storage.

IDENTIFICATION AND COMPARISON

  • Identification Process:
    • Use tests to confirm origins and exclude other sources.
    • Analyze gross physical properties and complex mixtures for evidence assessment.

CLASS AND INDIVIDUALIZATION

  • Class Evidence:
    • Groups evidence with similar characteristics.
    • Statistical analysis helps assign weight to evidence (e.g., blood typing).
  • Individualization:
    • Unique evidence associated with a high probability of common origin (e.g., fingerprint patterns).

TRACE EVIDENCE DATABASES

  • Database Types:
    • Investigative databanks for collection of reference samples (paints, hairs).
    • Statistical and interpretive databases to assess significance and characteristics of evidence.
  • Note: Trace evidence is rarely individualized; caution in assertions about origins.