Arson and Explosives

1
Arson and Explosives
2
Forensic Arson and Explosive Investigations

• Two Main Areas of Interest
• isolation and analysis of flammable residues
• collection and analysis of explosive material
• Any flammable liquid or substance used to start
  and/or maintain a fire is known as an accelerant
• Not all flammable or explosive material will burn
  in a fire accelerants always get trapped in
  pores in material and will not burn

3
Motives for Arson

• Revenge
• Vandalism (Boredom)
• Profit (Insurance Fraud)
• Concealment of a previous crime
• Terrorism
• Pyromania

4
The Chemistry of Fire
Fire is the product of combustion - the rapid
combination of oxygen with another substance
accompanied by the production of light and heat.

• The energy released comes from the breaking and
  reforming of the bonds that hold the compounds
  together

5
Ignition temperature

• Two Important Points
• Most combustion reactions will not happen by
  themselves (they are not spontaneous).they need
  help getting started
• Ignition temperature the minimum temperature at
  which a fuel will spontaneously ignite.

6
Stages of a fire

• Ignition
• Growth
• Limited to fuel items originally ignited
• Fuel controlled limited by fuel not oxygen
• Flashover
• Hot gases near ceiling reach 600ºC
• All items in room begin to burn
• Post-flashover steady-state burning
• Venillation controlled limited by oxygen
• Decay

7
Determining Point of Origin

• Lowest point that shows most intense burning
• Most intense burning can be determined from
  measuring char depth
• V-shaped pattern of burn will point to point of
  origin
• Light bulb burst in direction of highest
  temperature
• Point to origin

8
Indications of arson

• Lack of evidence of natural or accidental cause
• Evidence that a flammable liquid was among the
  first fuels to be ignited
• More severe burning on floor than ceiling implies
  accelerant
• Presence of accelerants
• Portable hydrocarbon detector - detects volatile
  organic residues
• Multiple points of origin

9
Accelerants and Explosives

• Accelerants and explosives are almost always
  organic in nature (wood, gasoline, kerosene,
  paper, etc)
• If an organic molecule burns completely, the
  products are always carbon dioxide and water.
  Smoke, ash, carbon monoxide result from
  incomplete combustion.
• Some materials must reach a temperature at which
  they decompose in order to burn (e.g. wood)
• Explosive materials are often compounds that are
  chemically unstable they want to react and form
  more stable molecules

10
Investigating Arson

• Steps to Follow
• Begin collecting evidence as soon as possible.
• Collect totally burned, partially burned, and
  unburned samples from the scene for comparison
• Back in the forensic lab, isolate and concentrate
  the accelerant material
• Headspace analysis
• Analyze the material to determine its composition
• GC or GC/MS

11
Gas Chromatograms
12
Explosives

• Explosives are substance that undergo a rapid
  exothermic oxidation reaction, with the
  production of a large quantities of gases.
• Often oxidizing agents
• Inorganic nitrate, chlorates and perchlorates
• High explosives
• Dont need to be confined to explode
• Detonation velocity gt 1000m/s
• TNT
• Low explosive
• Deflagration velocity lt 1000m/s
• Need to be confined or contained to explode

13
Low explosives

• Black powder
• Consists of charcoal and sulfur fuel and nitrate
  oxidant
• Smokeless powder
• Gun powder
• Singe-base contains only nitrocellulose
• Double-base contains nitroglycerine and
  nitrocellulose

14
High explosives

• Primary
• Sensitive to heat, shock, or friction
• Used to detonate main explosion
• Lead azide (PbN3)
• Secondary
• Relatively stable
• Will burn if small quantities ignited in open
• Dynamite
• TNT (trinitrotoluene)
• PETN (pentaerythritol tetranitrate)
• RDX (cyclotrimethylenetrinitramine)
• Tetryl (2,4,6-trinitrophenylmethylnitramine)

15
Home-made Explosives

• ANFO
• Ammonium nitrate soaked in fuel oil
• Believed to be used in
• World Trade bombing (pre 9/11)
• Oklahoma City bombing

16
Collecting Evidence

• Collect bomb pieces which are widely scattered
• Determine origin of blast
• A crater surrounded by debris
• Soil around origin is removed and stored in
  air-tight container
• Later washed with organic solvents to extract
  residues for analysis

17
Analysis of Explosives

• IMS - ion mobility spectrometry
• SEM-scanning electron microscope
• Color tests
• IR
• AA
• GC/MS