California officials quickly determined an arsonist started last month's huge wildfire southeast of Los Angeles, and that two weeks earlier sparks from a vehicle produced a deadly wildfire in the far northern part of the state.
But causes for many of California's worst blazes of the past decade remain a mystery.
The Associated Press reviewed state data on the 10 largest wildfires and 10 most destructive in terms of homes and buildings burned for each year dating to 2008. Lightning was the most common cause, accounting for about a quarter of those fires, followed by incidents involving power lines.
However, investigators could not determine a cause for about a third of those fires. Experts say each is a missed opportunity to learn something new.