By Diahann Reyes and Holly Yan
LOS ANGELES (CNN) -- It's already torched more than 35 square miles -- and the massive mountain fire in southern California is nowhere near under control.
The blaze, near Idyllwild and Fern Valley, prompted an evacuation order for about 6,000 residents, fire officials said. More than 4,000 homes south of Palm Springs are threatened by the fire.
Fewer than 5% of residents refused evacuation orders, officials said.
U.S. Forest Service spokeswoman Jeanne Pincha-Tulley said the blaze was "human-caused" and is under investigation.
As of Thursday evening, the inferno was about 15% contained. It has burned 22,800 acres -- up from the nearly 20,000 acres reported Wednesday night, officials said.
About 3,000 fire personnel were at the scene, U.S. Forest Service spokeswoman Melody Lardner said. Three firefighters suffered minor injuries Wednesday.
At least eight structures have been damaged or destroyed in the blaze.
CNN"s Amanda Watts and Miguel Marquez contributed to this report.