Investigators around San Diego continue searching for the causes of ten fires that burned thousands of acres of land in the area this week, after determining that one of the blazes was set by sparks from construction equipment.

Whether the other blazes were set intentionally or by accident, experts say it's highly likely that humans are to blame. Two people were arrested north of San Diego on Thursday on suspicion of arson, though it's not clear if they are thought to be connected to this week's big fires. (Related: What's Behind Early Season Winds Fueling Southern California Wildfires?")

Unlike remote parts of the world where natural events like lightning strikes are prime sources of wildfires, in southern California, such fires are