SAN DIEGO  — With evacuation orders being lifted Friday, investigators worked to determine whether an unusually early and intense outbreak of wildfires in Southern California this week was ignited by something as ordinary as sparks from cars or something as sinister as an arsonist.

State fire officials said the first of at least 10 blazes that broke out between Tuesday and Thursday was found to have been caused by a spark from malfunctioning construction equipment.

All together, the fires burned through more than 10,000 acres in the San Diego area, killing one person and causing more than $20 million in damage. At least eight houses, an 18-unit condominium complex and two businesses were destroyed, and tens of thousands of people were asked to leave their homes.

Six of the fires popped up within hours on Wednesday — raising suspicions that some had been set.