"Investigators believe the illegal campfire may have been smoldering for a few days until the strong winds blew embers into nearby brush," the agency said in a statement.
The fire broke out Wednesday morning and spread rapidly as flames were stoked by fierce Santa Ana winds in excess of 80 mph.
The powerful winds forced officials to ground aircraft and rely solely on a ground attack, with personnel in fire engines protecting homes and hand crews and bulldozers cutting fire breaks along the flanks of the fast-moving blaze.
The fire burned across the hillsides in Day Canyon above the Etiwanda Preserve, a remote area accessible only by narrow gravel roads.
More than 1,600 homes were temporarily evacuated and several schools closed after the blaze broke out about 8 a.m. above Rancho Cucamonga.
The fire spewed thick clouds of dark smoke into the sky and prompted officials to issue advisories for poor air quality.
Hundreds of firefighters from a number of agencies battled the blaze as temperatures hit the mid-90s and relative humidity dropped to a dangerously low 5%, officials said.
On Monday, the Forest Service said the blaze was 96% contained.