Crime Scene Processing: Key Steps, Documentation, and Chain of Custody

Securing and Isolating the Crime Scene

  • Purpose: ensure safety for responders and public; prevent further harm; protect potential victims; preserve evidence in a pristine state.
  • First principles:
    • Treat the scene as potentially active until proven safe.
    • Prioritize life and safety over evidence collection, then balance with preservation.
  • Immediate actions on arrival of first responders:
    • Determine that the scene is safe for responders and victims.
    • Render emergency medical aid as a priority, coordinating with EMS to minimize evidence destruction.
    • Observe and record bystanders and movements (watch the watchers; perpetrators may blend in).
  • Potential threats to watch for:
    • Gas leaks, poisonous substances, explosive materials, chemical, biological, or radiological threats.
  • Initial status and tasks:
    • May need to arrest suspects and conduct a preliminary sweep to ensure no hidden threats.
    • After medical aid and evacuation, EMTs should not clean up the scene; preserve it to avoid contamination.
  • Role transition once safety is established:
    • The lead investigator defines the crime scene boundary based on location, extent, and nature of the crime.
    • Larger boundaries are typically established first and may shrink later if needed.
  • Boundary control:
    • Establish a perimeter large enough to enclose the entire scene.
    • Exclude unauthorized people (bystanders, press, nonessential responders).
    • Use physical barriers (e.g., police line) and carefully log entry/exit of all people and materials.
  • Separation of individuals:
    • Suspects, victims, witnesses, medical personnel, and bystanders should be isolated for later questioning.
  • Key takeaway:
    • Securement and isolation are foundational; all subsequent steps depend on creating and maintaining a controlled environment.

Recording the Crime Scene

  • Comprehensive documentation is essential and scales with the scene’s size and nature.
  • What to document:
    • Location, conditions, and appearance of persons and items.
    • Weather conditions.
    • Locations of objects within the scene.
    • Identification of personal items.
    • Conditions of items (e.g., TV is warm, oven on, room smells of natural gas).
    • Preliminary witness interviews.
  • Methods to locate and record items:
    • Triangulation: uses two fixed reference points to determine object position.
    • Coordinate mapping: grid-based approach to map the scene.
    • Electronic methods: laser, GPS-based mapping; total station; CAD-based finished maps.
  • Triangulation details:
    • Let A(xA, yA) and B(xB, yB) be fixed reference points.
    • Object P is located at the intersection of two lines issued from A and B at measured angles θA and θB.
    • Parametric representations:
      extL<em>A(t)=(x</em>A+t cos⁡θ<em>A,  y</em>A+t sin⁡θ<em>A)ext{L}<em>A(t) = (x</em>A + t \, \cos\theta<em>A,\; y</em>A + t \, \sin\theta<em>A)extL</em>B(s)=(x<em>B+s cos⁡θ</em>B,  y<em>B+s sin⁡θ</em>B)ext{L}</em>B(s) = (x<em>B + s \, \cos\theta</em>B,\; y<em>B + s \, \sin\theta</em>B)
    • The coordinates of P are found by solving for t and s where \text{L}A(t) = \text{L}B(s).
  • Coordinate mapping details:
    • Scene divided into a grid of small squares (like a checkerboard).
    • Each square is searched and items are logged with grid coordinates to place them in context.
  • Electronic mapping details:
    • Technologies: laser scanning, GPS, total station.
    • Measurements are used to construct a detailed map starting from a rough sketch collected in the field.
    • CAD techniques produce 2D and 3D renderings; useful in court to illustrate the scene.
  • Photography:
    • Photographs create a permanent record of ephemeral evidence.
    • Photographers typically enter first to capture the scene in pristine state.
    • Photography approach:
    • Overview/wide-angle photos to show context.
    • Close-up photos to capture details.
  • Documentation goals:
    • Create a permanent, objective record of the scene and all evidence.
    • Support later analysis and presentation in court.

Searching for Evidence

  • Once mapped and recorded, investigators proceed to searching for relevant evidence.
  • The investigation lead decides:
    • The type of searches needed (scope, size, location).
    • The extent and location of searches.
  • Systematic search patterns:
    • Designed to ensure complete coverage and minimize missing areas.
    • Applicable to small interiors or large outdoor areas.
  • Practical challenge:
    • Distinguishing between background/environmental items and those relevant to the crime.
    • Experience helps; however, it is often better to collect more than to risk missing something important.
  • Protocols:
    • Police agencies develop extensive search protocols for consistency and legal defensibility.

Collection and Preservation of Evidence

  • When evidence is located, it must be:
    • Photographed in place.
    • Located on the scene map.
    • Collected and preserved properly.
  • Packaging considerations:
    • Different evidence types require different packaging to prevent contamination and degradation.
    • Volatile materials, liquids, and some controlled substances: airtight containers.
    • Moist biological evidence: placed in containers that facilitate drying (cardboard box or paper bag) to approximate natural drying conditions.
  • Goals of collection:
    • Minimize contamination and physical damage.
    • Maintain integrity from collection to laboratory analysis.

The Chain of Custody

  • Definition and purpose:
    • Chronological record of the custody of a piece of evidence from discovery to court admission and beyond.
    • Documents every transfer of the evidence and actions performed (analysis, storage, transport).
    • Ensures the evidence is truly from the crime scene and remains untampered.
  • Typical chain-of-custody documentation:
    • Evidence description and unique identifier.
    • Location found and date/time of discovery.
    • Person(s) who collected the evidence and every transfer point.
    • How the item was packaged and stored.
    • Current location of the item.
    • Any analyses performed and by whom.
  • Importance of continuous record:
    • A continuous, provable chain is required to prevent challenges to authenticity.
    • Breaks in the chain make evidence unlikely to be admissible in court.
  • Storage safeguards:
    • Evidence stored in a secure, locked location.
    • Not left unattended on lab benches or transferred unnecessarily many times.
  • Exceptions:
    • Some evidence may not require the usual chain-of-custody process if its uniqueness makes origin obvious (e.g., a serial-numbered object or a vehicle with detectable unique identifiers).
  • Practical implications:
    • Strict adherence to chain of custody is essential for admissibility and credibility.

First Responders: Roles, Safety, and Practical Implications

  • Responsibilities of first responders:
    • Ensure scene safety and provide medical aid when needed.
    • Preserve evidence by minimizing disturbance when possible.
    • Document movements of people and items by EMS to avoid contamination.
  • Ethical and legal considerations:
    • Balancing the urgency of medical aid with the need to preserve evidence.
    • Avoiding unnecessary exposure and protecting bystanders’ privacy where appropriate.
  • Practical outcomes:
    • Clear separation and documentation of roles help maintain scene integrity.
    • Proper early actions reduce risk of evidence loss or contamination later in the investigation.

Techniques and Concepts for Scene Mapping and Documentation

  • Rough sketch and final map workflow:
    • Field rough sketch with measured relationships.
    • Transfer to detailed CAD-based finished map.
    • Produce 2D and 3D renderings for courtroom visualization.
  • Mapping methods recap:
    • Triangulation (angles and fixed points).
    • Coordinate mapping (grid-based placement).
    • Electronic mapping (laser, GPS, total station).
  • Photography strategy:
    • Capture scene context first (overview).
    • Capture critical details up close (near and far).
  • Importance of technology in court:
    • Detailed maps and renderings help juries understand spatial relationships and sequence of events.

Practical and Legal Implications

  • Inadmissibility risks:
    • Inadmissible evidence can result from improper handling, contamination, or breaks in chain of custody.
  • Foundational principles:
    • Safety and triage take precedence but must be balanced with evidence preservation.
    • The integrity of the evidence and its chain is essential for legal admissibility.
  • Real-world relevance:
    • Proper training and adherence to protocols improve the likelihood of solving cases and securing convictions.

Key Takeaways

  • Crime scene processing is a structured, stepwise program: secure/isolate, record, search, collect/preserve, and maintain chain of custody.
  • First responders must prioritize safety and medical aid, then preserve the scene; watch for bystander dynamics and secondary hazards.
  • A well-defined boundary, controlled access, and careful documentation are critical to maintaining scene integrity.
  • Documentation is multi-modal: notes, photographs, sketches/maps, and formal records of movements and actions.
  • Evidence handling requires appropriate packaging to prevent contamination and to support later laboratory analysis.
  • The chain of custody is the backbone of evidentiary credibility; any breaks can compromise admissibility.
  • The use of triangulation, coordinate mapping, grid mapping, and electronic mapping (CAD/Total Station) enhances accuracy and courtroom presentation.
  • Ethical and practical considerations underline the need for meticulous, unbiased, and methodical processing of the scene.