It could take weeks or months for the public to know what caused the fire that last month charred 6,600 acres in the Otay Mountain Wilderness as local Cal Fire officials continue their investigation.
But it’s not just the Border 2 fire — the public still doesn’t have answers as to what has caused dozens of other wildfires in San Diego County from the past few years.
The vast majority, about 95%, of all wildland fires are caused by humans, said Cal Fire San Diego spokesperson Robert Johnson. But often the exact causes remain unknown — or are reported under a catch-all “other” category by Cal Fire.
The San Diego Union-Tribune reviewed Cal Fire’s incident data from 2019 through 2023, the most recent years for which comprehensive wildfire cause data are available. For nearly half of all fires in Cal Fire San Diego’s jurisdiction, no specific cause is reported to the public.
A quarter of those wildfires in San Diego County are recorded as having an “undetermined” cause. Another 20% of fires are categorized as “other,” meaning they don’t fall under one of Cal Fire’s categories of causes.
When asked, Cal Fire San Diego could not elaborate as to what “other” fire causes could include. In previous years, some fires classified under this category statewide have included ones started by fireworks, shooting and spontaneous combustion.
Wildfire investigations are often difficult to complete and take significant amounts of time and resources, Johnson said.
Cal Fire investigators study burn patterns to find the path traveled by the fire and the spot where the fire originated. Once they find it and determine the cause, they may decide they need to investigate further, by conducting interviews and collecting data and other evidence. That evidence could include matches, small pieces of metal, leftovers of fireworks, cigarettes, lighters or catalytic converter pieces.
If evidence is missing, or has been damaged or destroyed, that makes it harder for investigators to find the cause. The fire’s size, location and weather conditions all can also affect how long it takes to investigate.
“Investigations consider all possible causes of a fire until, based on their findings, they can determine the causes that are not a possibility or may be more probable than others,” Johnson said in an email.
Among all the fires in Cal Fire’s jurisdiction, the share of wildfires whose cause goes undetermined is significantly higher in San Diego County than elsewhere in California. Statewide, only 15% of those wildfires in the five-year span from 2019 through 2023 had no cause determined, and 16% of fires were categorized as “other.”
The single most common known cause of wildfires in San Diego County from 2019 through 2023 was vehicles, which account for 12% of fires during that time period. That can include fires ignited by cars, buses, motorcycles, off-road recreational vehicles, motor homes and trailers; fires could, for example, spark from a vehicle’s exhaust or from chains dragging on the ground.
About 11% of local fires have been caused by mechanical or electrical equipment, which could include sparks from welding, cutting or exhaust systems. That category doesn’t include electric utility equipment, such as power lines.
Arson accounts for 9% of local wildfires. It’s closely followed by open burning of trash, brush and the like, which accounts for 8% and could include, for example, prescribed burns that went awry or burning of yard debris.
About 5% of local wildfires have been caused by electric power generation, transmission or distribution equipment — lower than the statewide average of 8%.
San Diego Gas & Electric, the county’s electric utility company, has said it has not had a utility-related wildfire in 15 years — by which it says it means its equipment has not sparked a large wildfire of more than about 200 acres. The company has invested billions in fire prevention efforts, which include a network of weather stations and proactive power shutoffs during very dry and windy conditions.
SDG&E officials say sometimes its equipment contributes to a fire ignition because of incidents outside its control, such as when mylar balloons or birds interfere with the equipment or when vehicles run into it. And sometimes fires can spark because of a utility equipment failure.
Relatively few wildfires in San Diego County in recent years have been started by campfires or ceremonial fires (fewer than 3%), lightning strikes (fewer than 4%) or smoking (fewer than 1%).
