The Lowell Fire charred 2,200 acres by Tuesday, Cal Fire reported, which was 30 percent contained.
Cal Fire spokesman Daniel Berlant said crews hoped to fully contain the blaze by Aug. 1.
In addition, 1,800 structures in both counties remained under threat, and there were several mandatory evacuations still in place, Cal Fire said on its website.
Investigators have net yet determined the cause of the blaze, which was first reported Saturday about 2:30 p.m. in the Steep Hollow drainage west of Alta.
Four firefighters were injured in the blaze. U.S. Forest Service spokesman Stanton Florea said the federal agency still has one remaining firefighter in the burn unit at the University of California at Davis hospital. But he said two Cal Fire firefighters and one Forest Service firefighter had been released after battling a blaze racing through Nevada and Placer counties.