at the fire scene may become an issue for fire investigators in three
- The safety
risk of accidental secondary explosion caused by heating from a fire
- The safety
risk of secondary device explosion at an intentional bombing scene
- As a complication
of the fire investigation by an explosion prior to, concurrent with,
or immediately after a fire event
to a fire scene, first responders and emergency personnel should always
be alert for the potential of an explosive event. High-risk situations
and business fires with the possibility of explosion in natural gas
or LP gas supplying the building
fires that may touch off explosions of gas, oil, or other fluids in
the vehicle's mechanical system
industrial, and residential, fires where hazardous, flammable, and explosive
materials are present
where explosive dust may be found, such as coal dust, flour dust, saw
dust, or powdered fuel
(fireworks) factories and storage facilities-both legal and illegal
where a bombing is confirmed or suspected
reported by 911 calls or other witness accounts as having an explosive
component (i.e., heard a "bang" right before saw flames)
labs" where unknown chemicals and chemical combinations may be
responders should gather as much information as possible about the scene,
including asking tenants, owners, and neighbors about flammables and explosives
that may have been at the location. Utilities to the building should be
shut off as quickly as possible.
the risk of explosion is a concern, implement appropriate evacuation protocols
and scene security measures. At an explosion scene where a bombing is
a possible cause, a secondary devices sweep should be performed quickly.
Be aware that bombers have been known to place secondary devices in a
location where they expect emergency personnel to gather or evacuees to
congregate. Keep this in mind when choosing a location for the command
post and ensure that the command post area has been swept for secondary
devices, including a canine search for explosives. If any suspicious items
are found, evacuate the area and call the bomb squad.
an explosion scene, first responders and investigators must be alert for:
- The presence
of toxic materials
changes in substances at the scene (resulting from the heat of the fire
or the explosive event) that may have rendered them unstable or toxic
unexploded material or gases, including undetonated explosives, leaking
gases, and flammable liquids. Any unexploded material should be left
in place, evacuation measures taken, and the bomb squad or explosives
disposal team called.
Chicken or the Egg?
an explosion at the fire scene is confirmed, access to the location must
be limited. Establish a perimeter of no less than the distance of the
furthest piece of debris plus 50% of that distance from the blast seat.
Deploy law enforcement personnel at the perimeter to ensure no unauthorized
persons access the area. Designate a single point of entry and exit for
investigative personnel and log their movements. If new evidence is found
outside the perimeter, readjust the existing perimeter to the new furthest
piece of debris plus 50% of that distance from the blast seat.
fire investigator should be aware that explosions often leave very small
pieces of evidence, some of which may be nearly indistinguishable from
other, unconnected, materials in the room. For example, if the investigation
of a car fire involves a suspected bomb in the car trunk that also contained
a fax machine, it may not be readily apparent if a small electronic component
found in the blast seat is from the fax machine or from the device. Because
explosion evidence can be small and difficult to find, it is of increased
importance to control the scene and secure evidence areas so traffic that
may damage or move evidence is minimized.
an explosion is involved at the fire scene, one of the investigative tasks
becomes determining the nature of the explosion (accidental, intentional,
undetermined, or resulting from a natural event) as well as its cause-and-effect
relationship to the fire. Initially, the fire investigator may be charged
with making the determination of whether the explosion was accidental
or intentional. Trained and qualified fire investigators can eliminate
accidental explosion causes from the fire scene, as well as determine
whether the explosion caused the fire or the fire caused the explosion.
the bomb squad should be called immediately in cases where an explosive
device or suspicious package is found. In addition, the bomb squad may
also be tapped to respond to the scene in situations where an explosive
device is not found or immediately confirmed. If dispatch information
suggests that an improvised explosive device might be involved or that
the explosion is of unknown origin and criminal activity is a possibility,
the bomb squad can assist the investigator in determining what occurred.
In these cases, the bomb squad may even be called with first responders.
In addition, if, upon the investigator's arrival, the cause of an explosion
is not readily apparent, the investigator may request the assistance of
the bomb squad in determining the cause of the explosion and offering
expertise on whether there was criminal activity. The involvement of the
bomb squad will be guided by the response patterns in the investigator's
jurisdiction and standard operating procedures. In concept, the investigator
should be aware that s/he may be able to involve the bomb squad in investigative
activities, not just in crisis response to a suspicious package.
to "NFPA 921: Guide for Fire and Explosion Investigations 1998 Edition,"
the explosion that fire investigators will most frequently encounter is
a BLEVE (boiling liquid expanding vapor explosion). These types of explosions
involve containers holding liquids under pressure at temperatures above
their atmospheric boiling point. A BLEVE can occur when the liquid and
vapor in a containing vessel are exposed to the heat of a fire. The fire
raises the temperature in the vessel and increases the internal pressure
to the point where the container can no longer hold the gas and vapor.
The vessel ruptures and, if the contents are ignitable, combustion may
occur. BLEVE's do not have to be the result of heating; they can also
occur from mechanical damage or overfilling. 1
investigators may also encounter other types of mechanical explosions,
chemical explosions (usually mixtures of gases, vapors or dust with air)
and combustion explosions (from burning hydrocarbon fuels), and electrical
explosions (from a high-energy electrical arc or lightning).
to NFPA 921, when identifying whether the incident was a fire, an explosion,
or both and which came first, the investigator should determine:2
the damage is high-order (shattering effect, with generally smaller
pieces of debris) or low order (heaving effect with generally larger
pieces of debris). Also, the pattern of damage should be assessed throughout
the scene and used to inform the theory of what occurred.
- If the
explosion is seated or non-seated, where the origin was, and how the
origin is best described.
- The type
of explosion that occurred.
- What the
source and availability was of fuels according to the location and functioning
of utility services
- What the
ignition source was
- The distinction
between preblast and postblast fire damage. If debris that was hurled
out of the fire area is burned, that may indicate that the fire preceded
the explosion. Glass condition can also be an indicator: if glass from
a window broken out by the explosion has sooty residue, the fire may
have preceded the explosion. If the glass is pristine, the explosion
may have happened first.
making these determinations, answering questions like these will give
the investigator a "heads-up" on whether an explosion was accidental
or intentional and what the cause was:
- What indications
are there that the explosion occurred in an area that may have had combustible
or explosive materials present?
- What is
the layout of the utility systems and their delivery mechanisms vis
a vis the location of the explosion? Are there any conditions, such
as poorly maintained heating equipment, that may indicate the possibility
of accidental explosion?
- What sources
of fuel were available and how can they be eliminated as a cause?
- What ignition
sources were available and how can they be eliminated as a cause?
- Is there
a container that appears to have been involved in the explosion and,
if so, would it have been naturally in that location, or does it appear
to have been placed there?
- Is there
a potential target, human or physical, for an intentional bombing?
- Have all
potential accidental causes of the explosion been eliminated by the
fuel and ignition have been established, what is the theory of how the
explosion occurred? How is that theory supported by the physical evidence?
- What is
the timeline of the event?
- Is there
a defined blast seat? The lack of a blast seat may be an indicator of
a fuel gas explosion, pooled flammable liquid explosion, dust explosion,
or backdraft, because these types of explosions have large or diffuse
areas of origin, rather than a defined point, or are subsonic in nature.3
However, presence of a blast seat does NOT immediately indicate a bomb;
for example, a BLEVE may have a blast seat, as might an exploded boiler
or any explosion in a confined area.
- What evidence
is present and what does its damage pattern indicate?
- What injuries
are present on victims and how does this inform the chain of events?
examining these issues, the investigator must be wary of the exception.
For example, there have been fuel-air explosions that did not occur in
close proximity to the source of the gas. And, there have been bombs that
employed gas cylinders as containers but were not initiated by an accidental
explosive process. As with all good investigations, it is the totality
of the evidence that points to a conclusion, not a single factor.
Up the Pieces
the investigation indicates that a bombing may have occurred, it will
be prudent for the fire investigator to call in more specialized resources.
The investigation of bombings is a highly specialized area and many explosion
investigations are referred to federal agencies, such as the Bureau of
Alcohol, Tobacco, and Firearms. Your first call should be to
the closest public safety organization with explosive ordinance
disposal and/or investigation capability. Your local ATF Field Office
is also an excellent resource for explosives investigation. The Field
Offices are staffed by Certified Explosives Specialists (CES's) and members
of the Explosives Technology Branch (ETB) who are experts in the investigation
of bombings. Their personnel can be on your scene quickly to assist in
the investigation. These agents are skilled in component recognition,
blast effect interpretation, device reconstruction, and target analysis-all
of which will help bring the investigation to a successful conclusion.
NFPA 921 Guide to Fire and Explosion Investigations 1998 Edition. National
Fire Protection Association. Page 921-80.
2 NFPA 921 Guide to Fire and Explosion Investigations 1998
Edition. National Fire Protection Association. Page 921-90.
3 NFPA 921 Guide to Fire and Explosion Investigations 1998
Edition. National Fire Protection Association. Page 921-84.