The fire started Sunday afternoon in the Angeles National Forest above Glendora. It was 48 percent contained Wednesday and has burned more than 4,100 acres.
The U.S. Forest Service says a nearby mobile home park should be in the clear Wednesday as long as the fire continues to burn away from the property. Residents are expected to be able to return home on Thursday morning with proper ID.
Crews say it will take at least a week to fully contain the fire while ground crews carve out containment lines in the rough terrain.
Some firefighters from Australia and New Zealand were training Wednesday with Southland firefighters at the scene of the Williams Fire.
Highway 39 into the San Gabriel Canyon will be closed indefinitely as firefighters try to get a handle on the flames. The U.S. Forest Service says the road may reopen in a few days to residents only.
The cause of the fire was under investigation.