Fire Investigation, Explosives and Arson (lecture 19)

Fire Investigation

Behaviour of Fire

  • Point of Origin: Where a fire begins.
  • Fire Extension: Moves horizontally and vertically from the origin.
  • Necessary Factors: Fuel supply, oxygen, heat source.

Four Phases of Fire Progression

  1. Incipient: Earliest phase, very brief.
  2. Emergent Smoldering: Combustion products become noticeable.
  3. Free Burning: Increased rate/intensity of burning (intensity doubles with each 18°F increase).
  4. Oxygen-Regulated Smoldering: Oxygen depletion leads to dense smoke and potential backdraft when oxygen is reintroduced.

Fire Investigation Essentials

  • Documentation: Thoroughly document the scene.
  • Evidence: Acknowledge fire alters evidence; determine origin and cause related to victims.
  • Evidence Collection: Adequate sampling, photography, and autopsy for victim causes.

Origin and Cause Analysis

  • Evaluate Damage: Begin with less damaged areas to identify the area of origin.
  • Fire Movement: Fires typically move upwards; origin often at the lowest point of significant burning.
  • Fire Cause Categories:
    • Natural: Acts of nature (e.g., lightning).
    • Accidental: Unintentional causes.
    • Undetermined: Unknown causes.
    • Incendiary: Intentional fires.

Stages of Investigation

  • Examine both interior/exterior of structures.
  • Verify fire safety systems and compliance with fire codes.

Indicators of Arson

  • Signs of Arson: Look for accelerants, ignition devices, and typical signs like petroleum-based substances.
  • Myths: Many misconceptions exist regarding indicators of arson, e.g., char depth and burn patterns.

Evidence Collection Techniques

  • Sample Collection: Collect from points of origin using appropriate containers.
  • Gas Chromatography: Primary tool for analyzing flammable residues in laboratory settings.

Explosions

  • Defined by speed of reaction and gas production under pressure.
  • Types of Explosives:
    • Low Explosives: Black powder and smokeless powder; deflagration.
    • High Explosives: Sensitive, produce shock waves (detonation required).

Collection & Analysis of Explosives

  • Systematic search of bomb sites; preserve all items for lab examinations.
  • Use Ion Mobility Spectrometer for identifying explosive residues.