chapter 19

Arson and Explosives
Investigations

CHAPTER 19

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Learning Objectives, 1

Discuss the steps in the preliminary investigation of
arson

Be familiar with various types of burn indicators

Describe ignition devices that may be used in arson

Assess several common motivations of arsonists for
setting fires

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Learning Objectives, 2

Explain the scientific methods used in arson

investigation

List several groups of people whom an arson

investigator should interview

List questions that investigators should ask in

interviews and interrogations

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Learning Objectives, 3

Understand how legal entry can be made into a fire

scene and how evidence may be legally obtained

Discuss the two types of major explosions, namely,

mechanical and chemical

Understand how improvised explosive devices, I E

D’s, work

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Learning Objectives, 4

Outline the procedures for handling and

investigating bomb threats

Understand how to collect and preserve evidence at

the bomb scene

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Preliminary Investigation, 1

Premises should not be disarranged before an

investigation

Determine the point of origin by:

• Examining the bottoms of shelves, ledges, and moldings
• Examining the furniture and all sides of the legs, arms,

and framework of reconstructed furniture

• Using the layer-checking technique

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Preliminary Investigation, 2

Investigator should clean the floor carefully at the

point of origin, examining and moving all objects to

one side

• Floor or rugs should be swept as clean as possible for
examination of burn patterns

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Where and How Did the Fire Start, 1

Factors needed to cause a fire
• Source of heat
• Material ignited

Accidental fires
• Once the point of origin has been discovered, the next

step is to determine how the fire started
• Even though arson may be suspected, the investigator

must first investigate and rule out all possible accidental
or natural causes

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Where and How Did the Fire Start, 2

Causes of accidental or natural fires include:

• Electric system
• Electrical appliance and equipment
• Gas
• Heating units
• Sunlight
• Matches
• Smoking

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Spontaneous Heating

Produced in three major ways
• Chemical action
• Fermentation
• Oxidation

Susceptibility to spontaneous heating is usually
determined by drying time

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Spontaneous Ignition, 1

Rare in residences and small businesses

• Considerably accelerated by external heat such as:

• Sunshine
• Steampipes
• Hot air ducts
• Friction from wind or vibration

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Spontaneous Ignition, 2

Requires several hours to several months of
oxidation or fermentation

Characterized by internal charring of a mass of
combustibles

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Burn Indicators, 1

Effects of heat or partial burning that indicate the
following fire-related factors:

• Rate of development and points of origin
• Temperature and duration
• Time of occurrence and the presence of flammable

liquids

Determining the cause of a fire, especially arson,
requires the interpretation of burn indicators

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Burn Indicators, 2

Alligatoring

Depth of char

Breaking of glass

Collapsed furniture springs

Spalling

Temperature determination

Distorted light bulbs

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Burn Indicators: Depth of Char, 1

• Used for evaluating the
direction of fire spread
• By determining which

portions of a material were
exposed to the heat source
the longest

• By locating where the
damage was most severe
damage

• Used to estimate the
duration of a fire Source: Factory Mutual Engineering Corporation, Norwood,

Massachusetts.

Jump to Burn Indicators: Depth of Char, 1, Appendix

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Burn Indicators: Depth of Char, 2

The rate of charring of wood varies depending on
such variables as:

• Rate and duration of heating
• Ventilation effects
• Surface area-to-mass ratio
• Direction, orientation, and size of wood grain
• Species of wood, such as pine, oak, fir, and so on
• Moisture content
• Nature of surface coating

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Ignition Devices, 1

Matches
• Only juvenile arsonists and pyromaniacs seem to favor

striking matches

Gasoline and other accelerants
• Gasoline and other accelerants are very popular with

many different types of arsonists
• Accelerants: Boosters that speed the progress of a fire

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Ignition Devices, 2

Chemicals
• Various chemical combustions have been used to set

fires

Gas
• Use of a combination of gas and the pilot light on the

kitchen stoves of many residences is always a possibility

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Ignition Devices, 3

Electrical systems
• Any wiring system, including doorbell and telephone

circuits, can be used as a fire-setting tool

Mechanical devices
• Mechanical alarm clocks were once a favored weapon of

arsonists

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Ignition Devices, 4

Plants: Material placed around the ignition device
to feed the flame
• Newspapers, wood shavings, rags, clothing, curtains,

blankets, and cotton

Trailers: Used to spread a fire that has already been
started

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Arson for Profit, 1

Motive behind committing arson for profit is
economic gain
• Enormous gain derived from inflating insurance coverage

beyond the building’s value
• Limited economic gain derived from cutting one’s losses

before oncoming financial disaster

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Arson for Profit, 2

Factors that influence the insured person’s decision
to commit arson fraud
• Desire for financial relief
• Greed, the desire for easily obtained financial assistance

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Arson for Profit, 3

Reasons for arson
• Short-term business problem

• Desire to relocate or remodel
• Buildup of slow-moving inventory
• Outmoded technology

• Satisfaction of a legal or illegal debt
• Purely fraud schemes

• Redevelopment
• Building rehabilitation
• Real estate schemes
• Planned bankruptcy

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Major Forms of Third-Party Arson, 1

Elimination of business competition
• Motivated by someone who seeks to create a business

monopoly or at least to maintain a competitive edge

Extraction of extortion payments
• May be a warning signal to a businessperson to “pay up

or else,” or it may be a signal to similar businesstypes to
either pay or wind up like the burned-out victim

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Major Forms of Third-Party Arson, 2

Labor-management grievances
• Arsons in business establishments may be the result of

an unresolved labor-management grievance for which
the perpetrator felt there was insufficient redress or
resolution

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Revenge-Motivated Arson

Personal revenge: Use of fire to strike at an

individual owing to a personal grievance

Societal retaliation: Targets are random and fire-

setting behavior often escalates

Institutional retaliation: Often serial arsonists,

striking repeatedly at institutions

Group retaliation: Offender tends to feel anger

toward a group collectively

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Detection and Recovery of Fire-

Accelerant Residues, 1

After a fire has been extinguished, the floor should

be carefully cleaned

Baseboards and sills should be checked

Corners of the rooms should be checked

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Detection and Recovery of Fire-

Accelerant Residues, 2

Soil beneath the burned area should be checked

when gasoline or similar material is suspected to

have been thrown on porches or buildings without

basements

If recovered material is suspected of containing

flammable liquids, it should be sealed in an

uncoated metal paint can

Biodiesel fuel is one of the most common

alternative fuels used in debris analysis

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Detection of Fire Accelerants, 1

Olfactory detection
• Sensitivity of the human nose to gasoline vapor is about

one part per 10 million
• Sometimes, dogs are brought to the arson scene to

detect accelerants on people at the scene

Chemical color test detectors
• Chemical color tests may be used to detect both liquid

accelerant residues and their vapors

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Detection of Fire Accelerants, 2

Catalytic combustion detectors
• Portable, moderate in cost, and fairly simple to operate

Flame ionization detector
• Sample gas is mixed with hydrogen, and the mixture is

burned

Gas liquid chromatograph
• Sample gas is first separated into components on the

basis of the speed with which they travel through a tube
filled with packing material

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Detection of Fire Accelerants, 3

Infrared spectrophotometer
• Can achieve high specificity to flammable liquids and

high sensitivity

Ultraviolet fluorescence
• Consists of illuminating the darkened fire scene with an

ultraviolet lamp

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Interviews in an Arson Investigation, 1

Possible witnesses
• Tenants, businesspeople, and customers from the burnt

building or surrounding buildings
• Passersby such as bus drivers, taxi drivers, delivery

people, sanitation workers, police patrols, and people
waiting for buses and taxis

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Interviews in an Arson Investigation, 2

Questions to ask witnesses
• Was the fire observed?
• At what time and in what part of the building was the

fire observed?
• Was anyone seen leaving or entering the building before

the fire?
• If yes, ask questions relating to this person

• Were there any vehicles in the area of the fire?
• If yes, ask if they can be described

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Interviews in an Arson Investigation, 3

• Can the smoke and color of the flame be described?
• How quickly did the fire spread?
• Was the building burning in more than one place?
• Were any unusual odors detected?
• Was anything else observed?

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Interviews in an Arson Investigation, 4

Firefighters at the scene
• Can be an invaluable source of information to arson

investigators because of their technical knowledge
and what they observe at a fire

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Interviews in an Arson Investigation, 5

Questions to ask firefighters
• What time was the alarm received?
• At what time did the person arrive at the scene of the

fire?
• Was the route to the scene blocked?
• What was the extent of burning when the person

arrived?
• Were doors and windows locked?
• Were the entrances or passageways blocked?

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Interviews in an Arson Investigation, 6

• What kind of fire was it?
• What was the spread speed of the fire?
• In what areas did the fire start?
• How near was the fire to the roof?
• Was there evidence of the use of an accelerant?
• Was any evidence of arson recovered?
• Did the building have a sprinkler system?
• Were there contents in the building?
• What is the fire history of the building?

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Interviews in an Arson Investigation, 7

Insurance personnel
• Three people may be interviewed to determine if the

profit centers around an insurance claim
• Insurance agent or broker
• Insurance adjuster
• Insurance investigator

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Interviews in an Arson Investigation, 8

Questions to ask the agent or broker
• Who is the insured, and is there more than one person

insured?
• Is the insured the beneficiary?

Questions to ask the insurance claims adjuster
• Was a sworn statement taken from the insured?
• Did the insured submit documents regarding proof of

loss, value of contents, bills of lading, value of building,
and the like?

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Interviews in an Arson Investigation, 9

Questions to ask the insurance investigator
• Were you able to determine the cause of the fire?
• Did you collect any evidence?

Property Insurance Loss Register, P I L R
• Listing of everyone who has an insurable interest in fire

claims and a listing of only the insureds in burglary and
theft claims

Other witnesses concerning finances of the insured
• associates, creditors, and competitors

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Interviews in an Arson Investigation, 10

Questions to ask
• How long has he or she known the owner or insured?
• What is the nature of his or her relationship with the

owner or insured?

News media personnel
• Individuals affiliated with the print and electronic media

may have noticed something of value to the investigator
or have video footage of the fire and fire scene

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Interviews in an Arson Investigation, 11

Medical examiner
• Autopsy should reveal whether any victim found dead in

the fire was dead or alive before the fire started and
what the cause of death was

Interviewing a suspect
• Questions asked are based on the assumption that the

person to be interviewed is involved in arson for profit

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Interviews in an Arson Investigation, 12

• Questions to ask the suspect
• What method was used to accomplish the arson?
• Where did he or she obtain the incendiary device?

• Questions to ask the torch
• Is he or she willing to cooperate in the investigation?
• How many other people are involved in the arson-for-

profit scheme, how are they involved, and what role does
each person play?

• How did he or she first become involved in the plan?

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Interviews in an Arson Investigation, 13

Interviewing the target and the owner
• Target of the investigation may be an owner, landlord,

fire broker, or the like
• Questions to ask the target

• What does he or she know about this fire?
• When did he or she first hear of the arson?
• Where was he or she and what was he or she doing

before, during, and after the arson?

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Interviews in an Arson Investigation, 14

• Questions to ask the owner
• What does he or she know about this fire?
• How long has he or she owned the burned property?

Interviewing a potential informant who is not a
suspect
• Investigative efforts should be made to determine if the

informant has any police record
• If yes, could it have any bearing on the reliability of the

information provided?

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Arson Suspect

In most cases, a number of suspects emerge
• Investigator must determine which of the suspects had

the opportunity and the means to commit the crime
• Determination must be related to the background,

personal characteristics, past activities, and financial status
of each of the suspects

Direct evidence seldom links a suspect with a fire
• Investigator should concentrate on gathering

circumstantial evidence and some provable facts from
which valid conclusions can be drawn

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Photographing the Arson Scene, 1

Adverse conditions necessitate the use of camera

equipment that is reliable and quick

Photo session should take at least as long as the

physical examination

• Investigator should follow the same path in
photographing the structure as is followed in the

physical examination

Investigator should photograph things and areas

that show what happened in detail

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Photographing the Arson Scene, 2

In some cases, it is necessary to compile a

panoramic view

Area of the fire’s origin should be photographed

twice

• First before the rubble is disturbed and then after the
debris has been removed

If clocks are present, the investigator should always

photograph the faces of the clocks showing the

times they stopped

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Explosion

Physical reaction characterized by the presence of
four major elements
• High-pressure gas
• Confinement or restriction of the pressure
• Rapid production or release of that pressure
• Change or damage to the confining structure, container,

or vessel that is caused by the pressure release

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Explosives

Black powder or gunpowder

Nitroglycerin

Dynamite

Trinitrotoluene, or T N T

Plastic explosives

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Computerized Tomography, or C T

Method for generating a three-dimensional image

from multiple 2-D X-ray images captured from

multiple angles

Used to identify explosives

Large and expensive

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Sniffers

Called Ion Mobility Spectrometry, or I M S

Used in explosives detection

• Sample is heated to produce a vapor, which is then
ionized by a radiation source, something similar to

smoke detectors in a house

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Types of Explosions

Mechanical
• Explosions in which the high-pressure gas is produced by

purely physical reactions

• Example: Bursting of a steam boiler

Chemical
• Generation of high-pressure gas is the result of reactions

in which the fundamental chemical nature of the fuel is
changed

• Example: Burning of combustible hydrocarbon fuels

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Improvised Explosive Devices, or I E D’s

Combination of items or components that are

neither designed nor produced to be used in

conjunction with each other

• When they are placed together or assembled, they are
capable of exploding

Components

• Main charge explosive
• Fuzing system

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The United States as a Target of I E D’s

Boston Marathon bombers

The Times Square bomber

The Underwear bomber

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Investigating the Explosion Scene, 1

Objectives
• Determine the origin
• Identify the fuel and ignition sources
• Determine the cause
• Establish the responsibility for the incident

Without a preplanned, systematic approach,
explosion investigations become more difficult

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Investigating the Explosion Scene, 2

First duty of the investigator is to secure the scene
of the explosion

Investigator should establish a scene pattern
• Particular circumstances of the scene often dictate the

nature of the pattern

Areas can be searched more than once

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Investigating the Explosion Scene, 3

Structures that have suffered explosions are more
structurally damaged than those burned in a fire

Explosion scenes that involve bombings or
explosives have added dangers
• Investigators should be on the lookout for additional

devices and undetonated explosives

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Locating and Identifying Articles of
Evidence
Investigators should locate, identify, note, log,
photograph, and map any of the many and varied
articles of physical evidence

Investigators should keep in mind that significant
pieces of evidence may be found in a wide variety
of locations

Clothing of anyone injured in an explosion should
be obtained for examination and possible analysis

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Suspicious Packages and Letters, 1

Most bombers set and deliver the bombs
themselves

Precautions
• If delivered by a carrier, inspect for lumps, bulges, or

protrusions, without applying pressure
• If delivered by a carrier, do a balance-check to determine

if the package is lopsided or heavy-sided
• If there is a handwritten address or label from a

company, check if the company exists and if it sent the
package or letter

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Suspicious Packages and Letters, 2

Characteristics that could denote a suspicious
package or letter
• Packages wrapped in string
• Excess postage on small packages or letters, which

indicates that the object was not weighed at a post office
• Any foreign writing, address, or postage
• Handwritten notes
• Improper spelling of common names, places, or titles
• Generic or incorrect titles

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Suspicious Packages and Letters, 3

• Leaks, stains, or protruding wires, string, tape, and so on
• Hand delivery or a “drop-off for a friend”
• No return address or a nonsensical return address
• Delivery before or after a phone call from an unknown

person asking if the item was received

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