13.3 Wildfire Cause Determination

13.3 Wildfire Cause Determination

Section 27.10 of NFPA 921 (2024) governs cause determination for wildland fires, applying the same scientific-method requirements used for structure-fire cause work elsewhere in the standard. The investigator forms a hypothesis about the ignition source located within the ignition area (see Sec. 13.2), then tests that hypothesis against every piece of physical evidence, witness information, and circumstantial data collected — and rules out competing hypotheses rather than selecting the most convenient one. NFPA 921's long-standing prohibition on negative corpus — declaring a cause solely because all other causes were eliminated, with no supporting physical evidence for the cause actually assigned — applies with equal force in the wildland. An investigator cannot conclude "incendiary" merely because lightning, powerlines, and equipment were not found nearby; the ignition area must contain affirmative evidence consistent with the assigned cause.

Natural Causes

Only two natural ignition sources are recognized in wildland fire cause analysis: lightning and volcanic activity. Lightning is by far the more commonly encountered of the two in CFEI practice.

Lightning

Lightning is discharged static electricity associated with thunderstorm activity; the air temperature in a lightning channel can briefly reach roughly 50,000°F (about 27,760°C) — several times hotter than the surface of the sun — before cooling and contracting almost instantly. Because a lightning-ignited fire can smolder undetected for days, weeks, or even months before transitioning to open flame (a holdover fire), investigators should review lightning-detection data across an extended window — 30 days or more where weather and fuel conditions warrant — rather than only the day of the fire, and should cross-check multiple detection-network sources because sensor coverage and accuracy vary by network.

Field evidence of a lightning strike includes:

• Fulgurites — fused, glass-like formations created when lightning heats sandy soil or rock particles at the discharge point; typically hollow, branch-like, and fragile, requiring careful recovery.
• Tree damage — spike-top or blunted snags with no visible scar, tops blown off and scattered, needle shower, precipitated sap, spiral bark rips, and blowholes or disturbed soil at the base of the tree.
• Ground and low-vegetation effects — splintered limbs on low vegetation and small trees, shattered rocks, and disturbed soil at contact points.

Volcanic Activity

Volcanic ignition sources — ballistic projectiles, pyroclastic flows and surges, and direct lava contact — are geographically limited but follow the same evidentiary logic as lightning: physical evidence of the specific mechanism (deposited tephra, heat effects consistent with pyroclastic material, and similar findings) must be present at the ignition area, not merely inferred from regional volcanic activity.

Human-Caused Ignition Sources

Equipment and Vehicles

Internal combustion engines eject carbon and metal particles that remain thermally active long enough to ignite fine, dead fuels along roadways and off-road use areas:

A spark arrester or turbocharger reduces, but does not eliminate, the risk of particle ejection; malfunction, damage, or an incorrectly installed model can still allow a competent ignition source through. Beyond exhaust particles, equipment-caused ignition mechanisms include friction (metal fragments or heated particles torn loose when a hard object such as a rock or pavement is struck), fuel/lubricant/fluid leaks, mechanical breakdown, and radiant or conductive heat transfer from vegetation built up against a hot surface such as an exhaust manifold.

Power Lines and Electrical Utility Systems

Distribution lines (lower voltage, carrying power from the substation to the consumer) statistically cause more wildfires than higher-voltage transmission lines, largely because distribution networks contain more fire-starting hardware, more total line length, and closer proximity to vegetation. Documented ignition factors include equipment failure, deferred maintenance, high-hazard weather (high or turbulent wind, high ambient temperature), and vegetation or animal contact.

Conductor faulting can result from a broken or sagging conductor contacting the ground or vegetation, a failed splice or connector, proximity arcing (which can bridge open air to nearby vegetation at roughly 1 in. per 10 kV), or wind-driven line galloping or line slap; a fault that does not initially ignite a fire can still cause one later if the circuit automatically or manually recloses and re-energizes the line. Insulator failure — from dirt, salt deposits, bird excrement, over-voltage, lightning, or deliberate damage — can arc to the pole or crossarm and drop burning material to the ground below.

Field evidence consistent with a powerline cause includes downed conductors, arc pitting and staining on the conductor, blown fuses or open-position circuit breakers, resistance scarring on a contacted tree, blowholes at the tree base, and fulgurites where a conductor discharged directly to the ground — often at sandstone or other sandy-mineral surfaces.

Investigative Significance: Sec. 27.10 and supporting NWCG guidance impose a strict safety protocol before any powerline-cause fieldwork: treat every conductor as energized until the utility affirmatively confirms otherwise, never climb a pole, and wait for utility crews to make the line safe. Evidence collection, such as pole core samples, SCADA and maintenance records, and tree-contact samples, should be pursued only after the scene is confirmed safe and, typically, with a retained electrical engineer or powerline specialist.

Campfires, Debris Burning, and Smoking

Campfire escapes occur through direct flame impingement, aerial firebrands (paper and cardboard are common carriers), rolling burning material, creeping fire under or around a control barrier, or overheated rocks that shatter and eject burning embers. Improperly extinguished coals or ash can retain heat for hours to days; a crust of wetted ash on top can insulate smoldering material underneath, allowing the fire to escape well after apparent extinguishment. Circumstantial indicators include a fire ring, wood pile, seating, and other signs of recent camping activity near the ignition area.

Debris and open burning, along with smoking materials such as cigarettes and matches, are evaluated for competence as an ignition source using the same physical and environmental framework: a cigarette burns roughly 10–20 minutes at around 500°F in still air, with the glowing coal advancing toward its unlit end at roughly 0.21 in. per minute, giving any single point of fuel contact a heat-exposure window of only about 1–2 minutes. Investigators weigh this fact heavily when assessing whether a discarded cigarette alone was a competent ignition source, or whether an unusually intact cigarette instead suggests a time-delay device.

Incendiary (Arson) Determination

NFPA 921 distinguishes arson (a legal charge, defined by jurisdiction) from incendiary (the causal classification: a fire intentionally ignited in a place or under circumstances where a fire should not occur). Reaching an incendiary conclusion in the wildland follows the identical evidentiary discipline as every other cause discussed in this section: physical evidence of a deliberate, competent ignition mechanism located in the ignition area, not merely the absence of an accidental cause.

Only after cause is affirmatively established should an investigator consider motive, which the FBI classifies into six categories: retaliation/revenge, excitement, profit, vandalism, crime concealment, and extremism/terrorism. Investigating motive prematurely creates confirmation bias and risks focusing investigative resources on the wrong person; NFPA 921's broader 2024 guidance on confirmation bias applies directly here. Wildland arson investigators should also be alert to serial offending patterns (a suspect setting three or more fires with a cooling-off period between events) and spree patterns (three or more fires with no cooling-off period), both of which are best recognized through fire-history analysis and information-sharing across jurisdictions rather than offender "profiling."