California is in the midst of another devastating year of wildfires and finding the causes for the worst fires can be critical to identifying better fire-prevention techniques. But many times, investigators can’t figure out a cause. The Associated Press reviewed state data on the 10 largest and 10 most destructive wildfires for each year dating to 2008 and found investigators couldn’t identify the exact cause about a third of the time. Lightning accounted for about a quarter of those fires, followed by power lines. Among those where no cause has been determined yet are the Tubbs Fire that devastated Northern California’s wine country last year, and the Thomas Fire on the Central Coast, the second-largest in state history.