A brush fire just east of Los Angeles has grown to nearly 2,000 acres. 

Roughly 500 homes received mandatory evacuation orders and another 500 were urged to voluntarily evacuate.

They're all in and around the Riverside County city of Murrieta, which is about 70 miles southeast of Los Angeles.

Firefighters say the Tenaja fire took just a day to engulf nearly 2,000 acres.

Authorities say their biggest concern is what's called the Elsinore effect. This is when the fire dies down during the nighttime hours and then rapidly builds up again when the sun comes up.

Cal Fire Division Chief Todd Hopkins said "Eventually what will happen is around noontime to one o'clock, we get a basically 180 degree wind shift."

That reversal makes the fire more difficult for firefighters to control and could lead to more acreage burned.

Right now, officials say only two structures have been damaged.

Firefighters say the fire is about 10 percent contained.

The cause of the fire is still under investigation, but officials say it may have been started by a lightning strike.