"Specifically, a diesel-fueled vehicle emitting burning carbon from the exhaust system," the agency said. "The determination is reinforced by multiple independent witnesses, as well as supporting physical evidence."
The fire has destroyed at least four homes and two outbuildings while threatening dozens more, fire officials said.
As of Tuesday morning, the fire is 15 percent contained and has consumed some 41 square miles of dry brush and chaparral in Riverside County.
An emergency closure order was put in place Sunday prohibiting all entry into the San Gorgonio Wilderness Area, including the Pacific Crest Hiking Trail between the forest boundary and Forest Road 1N01, according to FOX11.
Several evacuations are in place in both Riverside and San Bernardino counties, impacting thousands of residents. At one point, sweeping orders impacted some 8,000 people.
John Medina, an American Red Cross spokesman, said that coronavirus precautions made for added stress at an evacuation center.
Volunteers used to "close contact” with evacuees have had to adjust their approach during a time of social distancing, Medina said.
“I mean, that’s part of the recovery of a disaster, is that you have to show warmth and love and caring. And that’s hard when you’re standing six feet away. So that’s the biggest challenge,” Medina told KESQ-TV.
The wildfire has generated a smoke plume that was visible for miles around and contributed to poor air quality.
Smoke from the wildfire has drifted over western Arizona and the Phoenix area.
Officials allowed flames from the Apple Fire to run up the side of Mount San Gorgonio, an 11,000-foot peak, because it wasn't safe to let crews work in such steep, rugged terrain, according to Lisa Cox, a spokeswoman for the U.S. Forest Service.
Firefighters battling the blaze have faced "dangerously hot conditions” that are expected to continue. Fire danger remains high on Tuesday for parts of the West.
Temperatures continue to soar over the Southwest with another day of excessive heat warnings.
The Southern High plains could also set records with daytime highs reaching over 100 degrees.
Fox News' Janice Dean and the Associated Press contributed to this report.