Fire debris as evidence

DEFINITION: Materials that have been partially or completely burned in fires.

SIGNIFICANCE: Fire debris is often the most valuable evidence at a fire scene. The analysis of fire debris evidence is used in fire investigations to help determine the causes and origins of fires. The results of the analysis of fire debris may indicate whether a fire was intentionally set or accidental.

Enormous amounts of debris are typically found at fire scenes. In an intense fire, the bulk of a structure can be reduced to soot and ashes. Sifting through such debris is a daunting task for fire investigators, but the work is worth the effort because the results of fire debris analysis often play a critical role in determining the cause and origin of a fire.

At the Fire Scene

One of the primary goals of a fire investigation is to identify exactly how the fire started. In many cases, fires are accidental, caused by electrical problems, gas leaks, or even lightning damage. In other cases, however, fires are set intentionally by people with malicious intent. Often the presence of an (a substance used intentionally to increase the rate and spread of a fire) indicates that a fire was purposely set. Accelerant residue is most likely to be found in the area where the fire started, or the origin.

A visual examination of the fire debris is typically the first step the investigator takes. By assessing the direction and intensity of the burn patterns, the investigator can identify the fire’s origin. A classic V-shaped pattern is often seen, as fires tend to burn in a tight area at the base and then spread as they move upward or away from the source. The V points back to the source of ignition, potentially an accelerant pool. Another typical burn pattern is known as a “trailer” or pour pattern. Arsonists often pour accelerant in a constant stream from room to room in an attempt to guarantee that the fire will spread throughout the entire structure.

Visual examinations of fire debris, however, are subjective in nature, as they rely primarily on the experience of individual fire investigators. Also, the damage may be too extreme for an investigator to identify burn patterns, or the patterns that are present may be misleading. Tools are available to assist investigators in the analysis of debris at a fire scene. Portable “sniffer” instruments are commercially available for the detection of hydrocarbon and organic vapors in the air. The air around the area suspected to contain an accelerant is drawn into the instrument, where any accelerant vapors are detected. These sniffers are sensitive but not selective. Many common household items emit hydrocarbon vapors when burned, and these may be identified by the sniffer. Another method of accelerant detection that is becoming increasingly popular is the use of trained dogs to sniff out accelerant odors. These dogs, known as accelerant-detection canines, undergo rigorous training and are less prone to incorrect identifications than are sniffing instruments.

Evidence Collection

After the origin of a fire has been determined, investigators can begin to collect to be taken to the forensics laboratory for further analysis. Debris is typically collected from several locations within the fire scene using everyday garden tools. If an area of heavy accelerant use has been identified, multiple types of debris from that area are collected. Most likely to contain traces of accelerant are porous surfaces, because accelerants can soak into them and be protected from the heat of the fire.

Different types of debris should be collected separately, even if they are from the same area. If several points of origin or areas of accelerant use are identified, representative debris samples should be collected from each area. In addition to debris from the areas suspected to contain accelerant residue, similar samples of debris that are not suspected to contain accelerant residue should be taken. These control samples are necessary to prove that any accelerant residue detected is not an inherent component in the debris material itself.

Fire debris evidence should be collected and stored in airtight containers. The majority of accelerants that may be present contain volatile components that may be lost in an unsealed environment. The preferred collection receptacle is an unused metal paint can with a friction lid. Glass jars are also acceptable, but only if they have airtight lids. Plastic containers should be avoided. As debris samples are collected, each container should be filled only about halfway, leaving an area above the sample empty. This space, known as the headspace, is needed for some extraction techniques used in the laboratory. Samples that are too large for the available containers should be cut and placed in several different containers. After the evidence has been collected, the containers should be stored in a temperature-regulated area to ensure that no volatile components evaporate.

Laboratory Analysis

In the laboratory, the samples are subjected to the standard procedures in place for fire debris analysis. Conventional techniques are used to extract residual accelerants from the debris. The choice of extraction procedure depends on the type of debris and on the potential accelerant used. Gas coupled with (GC-MS) is the standard analytical technique used for fire debris. The pattern of peaks present in the chromatogram and the chemical information available from the mass spectrum will help investigators to determine whether an accelerant, or any other significant component, is present in the fire debris.

Bibliography

Almirall, José R., and Kenneth G. Furton, eds. Analysis and Interpretation of Fire Scene Evidence. Boca Raton, Fla.: CRC Press, 2004.

Capistran, Briana and Edward Sisco. "Rapid GC-MS as a Screening Tool for Forensic Fire Debris Analysis." Forensic Chemistry, vol. 30, Sept. 2022, doi.org/10.1016/j.forc.2022.100435. Accessed 14 Aug. 2024.

DeHaan, John D. Kirk’s Fire Investigation. 6th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.

Nic DAÉid, Niamh, ed. Fire Investigation. Boca Raton, Fla.: CRC Press, 2004.

Redsicker, David R., and John J. O’Connor. Practical Fire and Arson Investigation. 2d ed. Boca Raton, Fla.: CRC Press, 1997.

Saferstein, Richard. Criminalistics: An Introduction to forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.