FORENSIC ASPECTS
OF ARSON AND
EXPLOSIVES
SFS4. Students will evaluate the role of
ballistics, tool marks and evidence of
arson in forensic investigation.
D. Evaluate possible indicators of
arson and criminal bombing.
INTRODUCTION
• Arson and explosions often
present complex and difficult
circumstances to investigate
because
• the perpetrator has, typically,
thoroughly planned the act
• the perpetrator is not present during
the act
• the destruction is so extensive
INTRODUCTION
• The criminalist’s function is rather
limited to…
• detecting and identifying relevant
chemical materials collected at the
scene
• reconstructing and identifying igniters
or detonating mechanisms
ARSON
DEFINITIONS
• Arson is the conflagration, or
uncontrolled burning, of any real
property without consent or with
unlawful intent
• Arson is prosecuted with attention to
degree of severity in the alleged
offense
• The analysis of fire-related
incidents is called fire investigation
THE CHEMISTRY OF FIRE
• Chemically, fire is a type of
oxidation, which is the
combination of oxygen with other
substances to produce new
substances.
• CH4 + 2O2  CO2 + 2H2O
• Oxidation reactions typically result
in large energy release, mainly as
heat and/or light.
THE FIRE TRIANGLE (or
TETRAHEDRON)
• Fuel
• Oxygen in
sufficient quantity
to combine with
the fuel.
• Sufficient heat to
initiate the
combustion and
sustain the
reaction.
THE CHEMISTRY OF FIRE
• The autoignition temperature of a
fuel is the minimum temperature
needed to spontaneously ignite it
without a spark or flame.
• White Phosphorus – 34°C
• Gasoline – 280°C
• Hydrogen – 530°C
THE CHEMISTRY OF FIRE
• The heat evolved when a
substance burns is known as heat
of combustion.
• Wood – 21.7 kJ/kg
• Gasoline – 47.3 kJ/kg
• Hydrogen – 141.8 kJ/kg
THE CHEMISTRY OF FIRE
• An additional factor, besides the
liberation of energy, needed to
explain fire is
• the rate or speed at which the
oxidation reaction takes place.
• A fuel will achieve a reaction rate
with oxygen sufficient to produce
a flame only when it is in the
gaseous state.
THE CHEMISTRY OF FIRE
• A liquid burns when the
temperature is high enough to
vaporize it – at the flash point.
• A solid must be hot enough to
decompose into gaseous
products – pyrolysis.
THE CHEMISTRY OF FIRE
• Glowing combustion or
smoldering is burning at the fuelair interface
• Cigarette embers, charcoal
• Spontaneous combustion, which is
rare, is the result of a natural heatproducing process in poorly
ventilated containers or areas.
THE FIRE SCENE
• The arson investigator needs to
begin examining a fire scene for
signs of arson as soon as the fire
has been extinguished.
• The necessity to begin an
immediate investigation even
takes precedence over the
requirement to obtain a search
warrant.
THE FIRE SCENE
• Experience shows that most arsons
are started with petroleum-based
accelerants.
• The search of the fire scene must
focus on finding the fire’s origin,
which may be most productive in
any search for an accelerant or
ignition device.
THE FIRE SCENE
• Some telltale signs of arson
include…
• evidence of separate and
unconnected fires
• use of “streamers” to spread the fire
from one area to another
• evidence of severe burning found on
the floor as opposed to the ceiling of
a structure, due to a flammable
liquid.
THE FIRE SCENE
• Normally, a fire has a tendency to
move in an upward direction, so
the probable origin will most likely
be the lowest point showing the
most intense characteristics of
burning.
• Fortunately, combustible liquids
are rarely entirely consumed
during a fire.
EVIDENCE COLLECTION
• At the suspected point of origin of
a fire, ash and soot, along with
porous materials which may
contain excess accelerant, should
be collected and stored in airtight
containers, leaving an airspace to
remove samples.
• It is important that a sampling of
similar but uncontaminated
control specimens be collected.
EVIDENCE COLLECTION
• Traces of flammable liquid
residues may be located with a
vapor detector (sniffer).
• Trained dogs can detect
1:1000000000 (1 part per billion)
EVIDENCE COLLECTION
• A search for igniters must also be
conducted.
• Matches
• An electrical sparking device
• Parts of a “Molotov cocktail”
EVIDENCE ANALYSIS
• In the laboratory, the gas
chromatograph is the most
sensitive and reliable instrument
for detecting and characterizing
flammable residues.
• The GC separates the chemical
components and produces a
chromatographic pattern
characteristic of a particular fuel.
EVIDENCE ANALYSIS
• By comparing select gas
chromatographic peaks
recovered from fire-scene debris
to known flammable liquids, a
forensic analyst may be able to
identify the accelerant used to
initiate the fire.
GC/MS ANALYSIS
GC ANALYSIS
Gas chromatograph of
vapor from a genuine
gasoline sample.
Gas chromatograph of
vapor from debris
recovered at a fire site.
EXPLOSIVES
DEFINITIONS
• An explosive is a reactive
substance that can produce an
explosion.
• An explosion is a rapid increase in
volume and release of energy in
an extreme manner, usually with
the generation of high
temperatures and the release of
gases.
EXPLOSIONS
• A sudden buildup of gas pressure
produces a shock wave.
TYPES OF EXPLOSIVES
• If the shock wave is subsonic, the
oxidation reaction is called a
deflagration
• The source explosive material is said
to be a “low explosive”
• If the shock wave is supersonic,
the oxidation reaction is called a
detonation
• The source explosive material is said
to be a “high explosive”
“LOW EXPLOSIVES”
• The most widely used are black
powder and smokeless powder.
• Black powder is a mixture of
potassium or sodium nitrate,
charcoal, and sulfur.
• Because it burns slowly, it is often
used as a fuse to a high explosive
• Smokeless powder consists of
nitrocellulose (plant fiber + H2SO4/HNO3)
• More stable, yet powerful, than BP
“HIGH EXPLOSIVES”
• They can be characterized by their
sensitivity to
• Impact
• Heat
• Friction
• Primary explosives are highly
sensitive
• Secondary explosives are fairly
insensitive
• Tertiary explosives are least sensitive
“HIGH EXPLOSIVES”
• Primary explosives provide the
major ingredients found in
blasting caps or primers used to
detonate other explosives.
• Nitroglycerin (in its pure form)
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“HIGH EXPLOSIVES”
• Secondary explosives will
normally burn rather than
detonate if ignited in small
quantities in the open air.
• Dynamite – composed of
nitroglycerin mixed with
diatomaceous earth
• Trinitrotoluene (TNT)
“HIGH EXPLOSIVES”
• Research Department Explosive (RDX)
• Composition C-4 – plasticized RDX
“HIGH EXPLOSIVES”
• Tertiary explosives, also called
blasting agents, are so insensitive
to shock that ignition by both
primary and secondary
explosives is required.
• Primarily used in large-scale mining
and construction and terrorism
• Ammonium Nitrate/Fuel Oil (AMFO)
EVIDENCE COLLECTION
• The entire explosion site must be
systematically searched with
great care given to recovering
any trace of a detonating
mechanism or any other item
foreign to the explosion site.
EVIDENCE COLLECTION
• Often a crater is located at the
origin and loose soil and other
debris must be preserved from its
interior for laboratory analysis.
EVIDENCE COLLECTION
• Debris and articles collected from
different areas are to be
packaged in separate air-tight
containers.
• It has been demonstrated that
some explosives can diffuse
through plastic and contaminate
nearby containers.
EVIDENCE ANALYSIS
• Typically, in the laboratory, debris
collected at explosion scenes will
be examined microscopically for
unconsumed explosive particles.
• Recovered debris may also be
thoroughly rinsed with organic
solvents and analyzed.
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EVIDENCE ANALYSIS
• Ion Mobility Spectrometry can be
used to preliminarily identify an
explosive residue
• Used in airports and by the US Army
in Improvised Explosive Device (IED)
examination.
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EVIDENCE ANALYSIS
• Confirmatory identification tests
may be performed on unexploded
materials by either infrared
spectrophotometry or X-ray
diffraction.
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