Cal Fire has officially ruled the arson. They are not releasing any further details while the case is under investigation.
The fire began a week ago on the Los Coyotes Indian Reservation near Warner Springs and spread to Anza-Borrego Desert State Park.
More than 2,100 firefighters from throughout California have been fighting the blaze.
The cost of the suppression effort has risen to $12.9 million.
The blaze began on land leased by the tribe to a private company called , which has provided the space to military and other government entities to conduct training. . The training center was founded by Brian Bonfiglio, a former vice president in San Diego for Blackwater Worldwide, a company that became controversial for its military security efforts in Iraq and other countries.
Warner Springs residents have complained about the possible fire risk of the activities.
Tribal police reported to Patch earlier this week that there had been a fire on the property months ago during training.
Click here to see a video tour of the property that was conducted by Channel 8 TV in May.
Cal Fire provided this map of the fire on Wednesday afternoon.
If you have any information concerning the arson, you may contact the Cal Fire Arson Hotline at 800-468-4408.