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Explosions at the Fire Scene

Cathleen Corbitt

Explosion at the fire scene may become an issue for fire investigators in three main ways:

  • 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

Safety First

When called to a fire scene, first responders and emergency personnel should always be alert for the potential of an explosive event. High-risk situations include:

  • Residential and business fires with the possibility of explosion in natural gas or LP gas supplying the building
  • Vehicle fires that may touch off explosions of gas, oil, or other fluids in the vehicle's mechanical system
  • Commercial, industrial, and residential, fires where hazardous, flammable, and explosive materials are present
  • Locations where explosive dust may be found, such as coal dust, flour dust, saw dust, or powdered fuel
  • Pyrotechnics (fireworks) factories and storage facilities-both legal and illegal
  • Scenes where a bombing is confirmed or suspected
  • Scenes reported by 911 calls or other witness accounts as having an explosive component (i.e., heard a "bang" right before saw flames)
  • "Drug labs" where unknown chemicals and chemical combinations may be potentially explosive

First 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.

If 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.

At an explosion scene, first responders and investigators must be alert for:

  • Structural damage
  • The presence of toxic materials
  • Chemical 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
  • Remaining 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.

The Chicken or the Egg?

If 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.

The 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.

Once 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.

Obviously, 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.

According 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

Fire 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).

According to NFPA 921, when identifying whether the incident was a fire, an explosion, or both and which came first, the investigator should determine:2

  • Whether 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.

In 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 on-scene investigation?
  • Given 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?

In 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.

Picking Up the Pieces

If 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.

1 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.

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