An illegal cooking fire at a homeless encampment was determined to be the cause of the Skirball fire, which began last week, authorities said Tuesday.

The fire was sparked just before 5 a.m. Dec. 6 in a brush area adjacent to where Sepulveda Boulevard crosses under the 405 Freeway, the Los Angeles Fire Department said.

The LAFD’s Arson-County-Terrorism Section investigated the fire and the results were released via email Tuesday afternoon.

“At the time of the LAFD’s arrival there were no individuals present at the area of origin,” LAFD spokesman Peter Sanders said in the release.

No arrests have been made in connection to the blaze.

The Skirball fire has burned 422 acres, destroyed six homes and damaged 12 others in the Bel-Air community.

The fire is 85 percent contained with 69 fire personnel remaining on scene to achieve 100 percent containment, fire officials said.

Additionally, no causes have yet been determined for the Creek, Rye and Lilac fires.

According to the National Park Service, about 90 percent of wildfires nationwide are human-caused.

In a statement issued Monday, Southern California Edison officials said they believe “the investigations now include the possible role of its facilities” after their personnel completed damage assessments in the Creek, Rye, Thomas and Liberty fires.

CalFire, other fire agencies and the California Public Utilities Commission were investigating the causes, Edison officials said.

“SCE continues to cooperate with the investigations,” their statement said. “The wildfire investigations may take a considerable amount of time to complete.”

An Edison spokesperson declined to comment further.

Despite Edison’s statement, Cal Fire is investigating all probable causes for the fires, not Edison specifically, said Cal Fire Deputy Chief Scott McLean.

“Nothing is off the board, and the investigation is not specific to them,” McLean said. “We’re very meticulous. It’s time consuming and very labor intensive.”

Nathan Judy, a spokesman for the Angeles National Forest, said human-caused fires are “always” a concern.

“It’s one of the reasons we have gone into extreme fire damage for the Angeles National Forest,” Judy said, explaining that campfires, stoves, lanterns or any other type of open-flame use are currently not allowed in any part of the forest.

The extreme fire designation will take place “for the foreseeable future until we get some significant amount of rain,” Judy said.

Angeles National Forest and CalFire officials said they are aware that transients live in and frequent the forest, adding that the human aspect remains a major concern for the authorities tasked with limiting the risk of wildfires.

Incidents of concern range from someone not watching their campfire to people driving through the forest with a trailer not realizing their chains were loose and causing sparks on the ground, Judy explained.