Forensic Aspects of Fire & Explosives Investigations

Fire Investigation

• Introduction
  • Arsons are complex to investigate due to:
  • Pre-planned crimes.
  • Absence of the perpetrator during the crime.
  • Extensive destruction at the scene.
  • Role of forensic scientists:
  • Detect and identify chemical materials.
  • Reconstruct igniter mechanisms.
  • Determining fire causes requires trained investigators with field experience.

Oxidation

• Fire as Oxidation:
  • Fire is defined as a rapid oxidation reaction that produces energy.
  • Not all oxidation leads to fire (e.g., rusting).
  • Exothermic Reactions:
  • Generates more energy than initially needed to start the reaction.
  • Energy forms: Heat and light are typical products of combustion.

Combustion

• Ignition Requirements:
  1. Fuel: Must be vaporized for a reaction with oxygen.
  2. Oxygen: Available in adequate quantity.
  3. Heat: Sufficient heat must be applied to initiate and sustain combustion.
• Heat of Combustion: Energy produced during burning.
• Chain Reaction: Energy released supports continuous combustion.

Combustion Triangle & Tetrahedron

• Combustion Triangle:

  • Oxygen
  • Heat
  • Fuel

• Fire Tetrahedron:

  • Oxygen
  • Heat
  • Fuel
  • Chain Reaction

Physical State of Fuel

• Gas: Reacts quickly to produce flames.
• Liquid: Requires high temperature (flash point) to vaporize for combustion.
• Solid: Needs high heat for pyrolysis to turn into gas.
• Smoldering: Occurs at fuel-air interfaces without flames.

The Fire Scene

• Investigators should examine fire scenes post-extinguishment for:
  • Signs of arson, often using petroleum-based accelerants.
  • Focus on the origin to find accelerants or ignition devices.

Indicators of Arson

• Typical signs include:
  • Unconnected fires or “streamers.”
  • Irregular patterns indicating accelerant use.
  • Severe floor burning suggests a flammable liquid.
  • Discovery of ignition devices (e.g., matches, cigarettes, explosives).

Collecting Fire Scene Evidence

• Collect ash, soot, and porous materials from origin into airtight containers (e.g., glass jars) – avoid plastic.
• Use vapor detectors and trained canines for detecting flammable residues.
• Perform control specimen sampling of similar uncontaminated materials from the scene.

Laboratory Recovery of Flammable Residues

• Heating Method:
  • Heat airtight container to drive off vapors.
  • Collect samples with syringes for GC analysis.
• Charcoal Strip Method:
  • Absorbs vapors during heating and analyzed in the lab.

Gas Chromatography

• Most sensitive tool for characterizing flammable residues.
• Compares chromatographic patterns of unknowns with known petroleum products.

Explosives

• Explosives Defined:
  • Substances undergoing rapid oxidation, producing gas pressure that leads to explosions.

Low Explosives

• Examples: Black powder and smokeless powder.
• Actions: Can be confined and promote deflagration (fragmentation of container).

High Explosives

• Primary Explosives: Sensitive to heat, shock, or friction (e.g., found in blasting caps).
• Secondary Explosives: Generally burn unless detonated by a primary explosive (e.g., dynamite, TNT, RDX).

Collection and Analysis of Explosives

• Use great care in recovering evidence at explosion sites.
• All items must be sealed in airtight containers to avoid contamination.

Lab Analysis of Explosion Evidence

• Microscopic examination for explosive particles.
• Analysis methods: color spot tests, TLC, GC/MS, IR spectrophotometry.

Summary Topics

• Introduction to fire and explosives investigation.
• Chemistry and nature of fire.
• Arson investigations and lab analysis of fire evidence.
• Details on explosives and explosion scene investigations.