LOS ANGELES -- The devastating blaze that broke out in the densely-populated Sepulveda Pass area last week was caused by a cooking fire, authorities announced Tuesday. The Los Angeles Fire Department stated that arson investigators determined the Skirball Fire -- which broke out in the early-morning hours of Dec. 6 -- was caused by a cooking fire at an encampment in a brush area near the Sepulveda Pass and the 405 Freeway.

CBS Los Angeles reports that when firefighters arrived on scene that morning, no one was found in the area, LAFD reports. No arrests have been made. It was unclear if the blaze was considered intentional in nature.

LAFD cites the National Park Service as saying that approximately 90 percent of wildfires nationwide are human-caused.

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The 422-acre Skirball Fire broke out at 4:50 a.m. Dec. 6 on the east side of the 405 Freeway near Mulholland Drive. It destroyed six homes and damaged 12 others, and at one time prompted the evacuation of about 700 homes and an apartment building. It also shut down the 405 Freeway for several hours.

One firefighter suffered neck burns and was treated at a hospital, authorities said. Another firefighter suffered minor injuries.

As of Tuesday, the fire was 85 percent contained, CBS Los Angeles reports. About 70 firefighters were still working to fully extinguish the blaze. All mandatory evacuation orders were lifted Sunday afternoon for all areas affected by the Skirball Fire. All road closures were also lifted, with no restrictions in place.

At its height, evacuation orders covered a 3.2-square-mile area bounded by Mulholland Drive to the north, Sunset Boulevard to the south, the San Diego Freeway to the west and Roscomare Road on the east. The exception to the evacuation order was the Bel-Air Crest housing development, which was not threatened, according to Los Angeles Mayor Eric Garcetti.

The Getty Center and the nearby Skirball Center, both on the west side of the freeway, reopened Friday, when classes at UCLA, Cal State Northridge, Los Angeles Valley College and Santa Monica College resumed.

All Los Angeles Unified School District schools in the San Fernando Valley and some on the west side of Los Angeles -- a total of 265 district schools and charter schools -- were closed Thursday and Friday.

The Santa Monica-Malibu Unified School District closed all of its schools Wednesday and Thursday.

The fire burned in the same general area as the devastating Bel-Air Fire of 1961. That blaze destroyed about 500 homes and led to various policy changes, including a prohibition on wood-shingle roofs and the strict requirement to clear brush around properties.