A state fire official says a spark from construction equipment ignited the first of 10 major fires that ignited in San Diego County this week.
California Department of Forestry and Fire Protection spokeswoman Lynne Tolmachoff said Friday that investigators believe the blaze that charred more than 1,500 acres in the Rancho Bernardo suburb was accidental.
She says causes of the other fires have not been determined. Officials are looking into everything from car sparks to arson as possibilities.
The Rancho Bernardo fire started Tuesday and was largely under control by Wednesday when eight more fires erupted.