A fire investigation is only as strong as the evidence that supports it. Items recovered from a fire scene may be examined by a crime laboratory, an engineer or a manufacturer's representative, and they may be scrutinized in court years later. If evidence is poorly documented, contaminated or lost, even a correct conclusion can be undermined. At the 2018 Utah Chapter IAAI conference, sessions on "Evidence Collection: What to collect and how to collect it" and on proper procedures for submitting evidence to the state crime lab were paired with burn cell exercises for exactly this reason. This article outlines the fundamentals.
Start With a Plan
Before collecting anything, consider what questions the evidence needs to answer. Evidence collection should follow your origin analysis, not replace it. Ask:
- What hypotheses about origin and cause am I testing?
- Which items could confirm or eliminate each hypothesis?
- Which laboratory or expert will examine the item, and what are their packaging requirements?
- Are there other interested parties, such as property owners, insurers or manufacturers, who should be notified before items are moved or altered?
Document Before You Move Anything
Every item should be photographed in place, with and without a scale, and its location recorded on a diagram with measurements to fixed reference points. Record who found it, when and under what circumstances. Our fire scene photography guide explains how to photograph evidence in context. Only after documentation is complete should the item be collected.
Common Types of Fire Scene Evidence
Fire debris for ignitable liquid residue analysis
When an ignitable liquid is suspected, samples of debris, flooring, carpet and padding, soil or porous materials from the suspected area are collected for laboratory analysis. Liquids soak into porous materials and protected areas, such as seams, cracks and the underside of flooring, so these are often productive sampling locations. Detection canine alerts can help select sample locations, but NFPA 921 states that canine alerts should be confirmed by laboratory analysis.
Comparison samples
Many materials contain compounds that can resemble ignitable liquid components. Collecting a comparison sample of the same material from an area unlikely to have been exposed to an ignitable liquid helps the laboratory interpret results.
Electrical and appliance evidence
Conductors with arc sites, circuit protection devices, receptacles, switches, cords and appliances may be critical to testing electrical hypotheses. Document their routing and connections before removal. When appliances or products may be involved in civil claims, consider leaving them in place until interested parties can attend a joint examination. See investigating electrical fires.
Physical, trace and digital evidence
Containers, ignition devices, tools, footwear impressions, tool marks and fingerprints may be important in incendiary cases. Digital evidence, such as security camera recordings, alarm system logs and smart device data, should be identified and preserved quickly because it can be overwritten.
Choosing the Right Container
Volatile compounds are easily lost or contaminated, so containers matter:
- Clean, unused metal cans with tight-fitting lids are commonly used for fire debris. Leave headspace; laboratories commonly recommend filling cans no more than about two-thirds full.
- Glass jars with appropriate lids can be used for small samples and liquids.
- Specialized evidence bags designed for fire debris can be used for larger or irregular items.
- Ordinary plastic bags are generally unsuitable for debris suspected of containing ignitable liquid residues, because volatile compounds can be lost through them or the plastic may contribute contaminants.
Always follow the receiving laboratory's current guidance. The IAAI's online Fire Scene Evidence Collection Guide is a helpful reference for packaging specific item types.
Preventing Contamination
- Change gloves between samples and use clean tools, or clean them between samples according to your agency's procedure.
- Keep fuel-powered equipment, such as saws, generators and fans, away from sampling areas where possible, and document its use.
- Do not store evidence containers where they could be exposed to gasoline or solvents, including vehicle trunks near fuel cans.
- Consider collecting a sample of unused gloves or packaging materials when required by your laboratory's procedures.
Labeling and Chain of Custody
Each container should be sealed and labeled with a unique item number, a description, the location where it was found, the date and time, and the collector's name. Chain-of-custody documentation must show every person who has had custody of the item, and when, from collection until the item is presented in court or released. Gaps in that record invite challenges. Standards from ASTM International, such as ASTM E1459 on physical evidence labeling and ASTM E1188 on collection and preservation of information and physical items, are widely referenced in fire investigation.
Spoliation and Interested Parties
Spoliation is the loss, destruction or material alteration of evidence that may be relevant to litigation. It can harm criminal and civil cases alike. NFPA 921 discusses spoliation and the importance of notifying interested parties before evidence is altered or destructive testing occurs. Private-sector investigators in particular should be familiar with ASTM E860, which addresses examining and preparing items that are or may become involved in litigation.
Submitting Evidence to the Laboratory
- Confirm the laboratory accepts the evidence type and what analyses it can perform.
- Complete the laboratory's request form, clearly stating the questions you want answered.
- Package and seal each item separately; do not overfill containers.
- Transport and submit evidence promptly, and store it securely until then.
- Record the transfer on your chain-of-custody form and file the laboratory's receipt.
Laboratory results are data, not conclusions. They must be interpreted together with all other information in your analysis, as described in our article on NFPA 921 and the scientific method.
Common Evidence Mistakes
- Collecting items before the origin has been established, which can destroy the very patterns needed to interpret them.
- Failing to collect comparison samples, leaving laboratory results difficult to interpret.
- Combining several items in one container, making it impossible to say where a result came from.
- Leaving containers unsealed or unlabeled "for a few minutes" on a busy scene.
- Not recording who else handled the item, or when it was released.
Building Evidence Skills
Evidence collection is a hands-on skill. The IAAI offers the IAAI-ECT (Evidence Collection Technician) credential, described in our certification guide, and the Utah Chapter includes practical evidence exercises at its annual conference when possible. Check the Training page for upcoming opportunities and browse more investigator resources.
This article is general educational information. Follow your agency's evidence policies, your laboratory's submission requirements and applicable law.