Part investigative science and part forensic science, the whole field of forensic arson/explosion investigation encompasses a variety of people in various positions -- police arson and bomb squads; firefighters and fire marshals; safety professionals, insurance and private investigators; forensic chemists and other criminalists; civil, electrical, and mechanical engineers. The United States invests as much, if not more, resources in the "war on fire" as it does in the "war on crime", so it's not surprising that many experts in this field are government employees. Arson investigators sometimes think of themselves as counterparts to homicide investigators (arson as murder by fire), and explosives experts (who may think of themselves as involved in counter-terrorism) usually fall into one of two types: police or laboratory personnel.

    It's difficult to say who's the forensic scientist in this field, that is, the one who testifies as an expert on the stand. Police bomb squad duties usually consist of mostly retrieval and disposal, responsibilities more in common with HAZMAT than forensic arson/explosion investigation. Likewise, many firefighters don't have the specialized talents of arson investigators. Add to this the problems of jurisdiction and overlap between police and fire departments, and you begin to see the confusion. A lot depends upon job title and work-related responsibilities. In this forensic area, job function determines expertise.

    Fires and explosions, by their very nature, require extensive on-site investigation, so it's a safe bet to say that the one who testifies as the forensic expert will be the one who is most able to make sense of chemical tests conducted by criminalists AND has practical on-site investigative knowledge of the crime scene.  In most cases, this will be someone called the arson investigator, although explosion experts can be found separately. In rare cases, forensic chemists, pathologists, and other experts also testify.

    Any expert witness in this area should have an understanding of the chemistry of fire and the principles of combustion. They should not only keep up-to-date with the research literature, but they should have actual experience in the handling of explosives. Few fire safety education programs exist, so forensic law doesn't require any college degrees (although it's a plus). Both basic and advanced training courses exist at various institutes, and these are commonly sought-after credentials. Laboratory personnel must usually be qualified at recognizing chromatographic patterns. Experts are also generally expected to be familiar with manufacturer and association tests and standards, not that there are any proficiency tests, but that belonging to a professional association goes a long way in qualifying as an expert and in keeping one's skills sharp.


    All forms of fire and explosion are subtypes of the larger term, combustion. Fire is a fundamental chemical reaction based on oxidation. Explosion is a two-tiered chemical reaction based on the volatility of mixing at least two substances.  The following list of definitions might be helpful:

    Heat is one of the most basic terms. It is produced from the breaking and formation of chemical bonds. In a chemical reaction, atoms are not lost but merely redistributed. Molecules absorb energy when their chemical bonds are broken apart, and release energy when their bonds are reformed. All oxidation reactions give off more energy than they absorb, which is released in the form of heat, light, shock, noise or some combination thereof, depending upon the single- or double-bonding that takes place when the molecules reform. Most reactions are exothermic, which means they need very little energy to get started (a lower ignition or kindling temperature), and some reactions are endothermic, which means they need more energy to get started (a booster).  

    Reactions also take place at various rates of speed. Fire, for example, is a fairly slow reaction because molecular change usually only takes place on the surface of substances (this is called glow as opposed to pyrolysis which is flame and indicates irreversible molecular change). Pyrolysis only takes place when fire has a continuous source of oxygen. Fuel-air mix determines what is called the flammable range, and below this is the ignition range, and even further below is the flash point (vapor) range. Conditions are right to support combustion (give an outside source) at the flash point range, reactions will sustain themselves at the ignition range, and fuel-air mix is perfect at the flammable range. How quickly reactions move through these three ranges is the speed of the reaction.  Speed can be increased by temperature, however, as any 18 degree Fahrenheit increase in temperature usually doubles or triples the reaction rate. Fires, for example, burn faster once they can raise the environment's temperature.

    The earth's atmosphere consists of 21% oxygen, and most fires will extinguish themselves at less than 16% oxygen.  However, it's not a simple matter as the external supply of oxygen. Chemical reactions have their own way of extracting the oxygen they need. With a fire involving hydrocarbons (wood), the reaction extracts oxygen from carbon dioxide, carbon monoxide, sulfur oxide, and nitrogen oxide. With a fire involving plastic, the reaction relies on poisonous gases released: hydrogen cyanide, hydrogen chloride, and phosgene. Explosions rely upon oxygen released from nitrates or potassium substances, usually potassium nitrate, potassium phosphate, or nitrogen itself. Much of what chemical reactions need to supply their own oxygen is abundantly available in the air (carbon dioxide) or nature (nitrogen and potassium). These readily-available substances are called oxidizers.

Fire Triangle:

Explosion Triangle:

(Please review the Lecture on Arson Law and Lecture on Arson Profiles)

    Step #1: Determine that the fire is of "suspicious origin" and merits a case-solving investigation. Although there's some departmental variation in terminology, classification is as follows: a) natural; b) accidental; c) unknown origin; d) suspicious; e) incendiary. Obvious arsons are in the last category. 

    Step #2: Photograph the "burning pattern" by tracking damage and by drawing sketches. All fires burn upward in an inverted conical shape. The point of origin is usually therefore the lowest place. Track from least damage to most damage.

    Step #3: Theorize, calculate, and estimate time from ignition to flashover.  You want to get some estimates of the volume of heat.

    Step #4: Develop leads as to who had motive, opportunity, and means.

    Step #5: Research backgrounds of suspects (for their technical know-how).

    Step #6: Put main suspect under surveillance, use informants, or interrogate suspect(s).

    Arson fires are generally indicated by: burning faster and larger than normal; often having multiple points of origin; use of accelerants; a time-delay device (a matchbook and cigarette is a 22 minute fuse); and cans and containers sometimes nearby. Odors are more important to trace than charred wiring. Planned fires are designed to have the nearby presence of ventilation, combustible material, and sometimes accelerant.  Firemen are trained to "overhaul" the structure by ripping out cabinets, spaces between studs, etc. to help investigator.  Heat can also be estimated by the condition of window glass: small shards of glass means an explosion took place; "crazing" means a hot fire. Melted copper, aluminum, and other metals usually means an accelerant was used. Soot that wipes off easily from glass or slightly charred studs behind the walls means a quick fire. Disgruntled ex-employees, ex-renters, transients, and juveniles are the usual suspects, unless you've got a professional arsonist on your hands.


    Bombs can be made very simple or very sophisticated, depending upon how much chemistry the suspect knows. If your suspect is fairly unsophisticated and just wants to make a quick bomb, then they will make a "pipe bomb" where they simply cracked open some shotgun shells, put the black powder in a pipe, closed the ends, and added a fuse. What happens with a pipe bomb is that the explosion, being confined to a sealed container, produces a large volume of gas that actually causes the walls of the pipe to balloon and stretch until broken pieces of shrapnel fly out. Gas compression bombs of this nature have a strong shock wave effect, to the order of 7000 mph. More serious damage is done by the blast effect than the fragmentation effect. It's like having a hurricane in a box.

    Bombs can be classified as "low explosives" or "high explosives" depending upon the speed of detonation (which is just the chemical reaction time). With a low explosive (like black powder), the shock wave is only about 2300 meters per second, no primer is really needed, and the bomb is sensitive to heat, friction, and temperature (so don't drop it).  With a high explosive (like TNT), the shock wave is as high as 6900 meters per second, some kind of primer or blasting cap is needed, and the bomb is relatively insensitive to heat, friction, and temperature (although don't play football with it). Low explosive materials are only lethal when they are confined in a container. If you sealed natural gas, a gasoline/air mixture, charcoal, sulfur, starch, phosphorous, magnesium, or just about any other household commodity in a container, you would have a low explosives bomb. Mixtures in the "lean" range (20% oxygen or more) generally explode more than create a fire. Mixtures in the "rich" range (10% oxygen or less) generally explode, then suck surrounding air in (with a "whoosh" sound), creating more destructive fire than explosion.

    High explosives (like TNT, RDX, and PETN) are available only commercially or militarily. Dynamite, however, has been around since Alfred Nobel discovered it in 1890, but his nitroglycerin-based methods have been replaced in recent years by ammonium-based or emulsion-based methods. Nobel's dynamite was a rather solid mixture of 78% sodium nitrate and 12% nitroglycerin; modern dynamite is a gel that mixes oxygen-rich ammonium nitrate with guar gum or uses water/oil droplets in a hydrocarbon base with micron-sized glass, resin, or ceramic microspheres. That's why one of the modern terms for dynamite is "gelignite" and although it still causes headaches when absorbed through the skin, it is less water soluble. Ammonium nitrate is a common fertilizer. When mixed with fuel oil, it's called ANFO, and this was the kind of explosive used on New York's World Trade Center in 1993.

    TNT is also unaffected by moisture (because it's sealed in wax), and is the stuff inside of WW II-era bombs, shells, and grenades. Nitroglycerin is derived from Toluene (a petroleum by-product), hence it's name Trinitrotoluene. TNT is trinitrotoluene while nitroglycerine is glyceroltrinitrate, derived from glycerin. Nitroglycerine is extremely shock-sensitive and unpredictable. TNT is nitrated toluene, and is much more stable. It is more closely related to the nitrated phenols.

    RDX, a British invention during WW II (standing for Research Dept. Explosive) is what the military uses today. Sometimes, they mix RDX and TNT in what is called Cyclotol or C-6, but RDX alone is more commonly used and called C-4 (for military designation Composition 4). It has a very plastic, dough-like consistency and charges can be shaped for special detonation effects. Breathing the dust off of it can cause epilepsy and amnesia.

    PETN (Pentaerythritol Tetranitrate) is a nitrate ester. "Esters" are a special kind of substance created in chemistry labs in a distillation process not unlike making alcohol. They belong to a class of chemicals called carboxylic acid derivatives, and in the scheme of things, they are classified between amides and anhydrides (amide < ester < anhydride < halide).  The distinguishing feature of PETN is the ring size at the molecular level (Oxy -1, Oxy-2, Oxy-3, or Oxy-4). It is used in "primacord" which can also shape charges. 

    Bombs in packages, suitcases, boxes, etc. are usually triggered by a battery-powered switching mechanism (clock, mercury switch) which is activated when the package is opened. A few bombers prefer a more steady, reliable external source. Car bombs, for example, are usually powered by the vehicle's ignition switch. Most of whatever ingenuity is invested in the bombing by the bomber goes into the switching mechanism.

    Color spot tests exist for most common volatile materials. X-ray examination and infrared spectrophotometry will reveal most organic explosives. Chromatography is necessary for detecting traces of plastic or military-grade explosives.

ATF Explosives Group
ATF List of Explosive Materials

C.I.S. Fire & Arson Investigations Website

EKU's Fire and Safety Degree Program

Fire and Arson Investigation Resource Page

IAAI's Fire Investigator's Checklist

Pyrotechnics FAQ

Strawberry Pop-Tarts as Incendiary Devices

T.C. Forensic

Technologies for Detecting Bombs & Explosives

Volatile Organic Compounds

The Terrorist's Handbook (web edition)
DeHaan, J. (1991). Kirk's Fire Investigation, 3rd ed. Englewood Cliffs: Prentice-Hall.
Midkiff, C. (1982). "Arson and Explosion Investigation" in R. Saferstein (ed.) Forensic Science Handbook, Englewood Cliffs: Prentice-Hall.
Moenssens, Starrs, Henderson & F. Inbau. (1995). Scientific Evidence in Civil and Criminal Cases. Westbury, NY: Foundation Press.
Pickett, M. (1998). Explosives Identification Guide for First Responders. NY: Delmar.
Redsicker, D. & J. O'Connor. (1996). Practical Fire and Arson Investigation. Boca Raton: CRC Press.
Saferstein, R. (1998). Criminalistics: An Introduction to Forensic Science. NJ: Prentice-Hall.
Yinon, J. & S. Zitrin. (1996). Modern Methods and Applications in Analysis of Explosives. NY: Wiley.

Last updated: 05/26/04
Syllabus for JUS 425
MegaLinks in Criminal Justice