A lightning strike likely ignited a 2020 wildfire in California’s Sierra National Forest, but the U.S. Forest Service said Friday it could not determine an official cause of the fire.

September’s Creek Fire burned 600 square miles and spread so quickly that hundreds of Labor Day holiday campers had to be rescued by a series of harrowing helicopter flights. All 214 campers were delivered safely.

Investigators did not rule out arson and lit cigarettes as the cause, but said there were no illegal marijuana grow sites nearby that could have started the fire. Forest service officials said an “undetermined” status is not uncommon with a fire this complex.

“Investigators spent countless hours hiking rugged terrain to determine the cause, interviewed numerous leads, and eliminated multiple potential causes. In the end, lightning remains as the probable cause,” said Dean Gould, forest supervisor of the Sierra National Forest.

Fresno County Supervisor Nathan Magsig expressed gratitude for the service completing its investigation but said an undetermined cause makes it “hard for the residents and those who lost so much to find closure.”

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