(CNN)Despite an "exhaustive investigation," the US Forest Service announced Friday the cause of the largest single fire in California's recorded history is undetermined.
The Creek Fire, which burned more than 379,000 acres in the mountains of Fresno and Madera counties for months last year, destroyed more than 850 structures and spawned two firenados amid the state's worst fire season on record.
Lightning is the most probable cause of the wildfire, but authorities aren't ruling out arson or "smoking activities," fire investigators said in a news release. The undetermined status is "not uncommon given the complexities involved," officials said in a news release.
Officials excluded many potential causes such as equipment use, escaped campfire, activities by children and burning debris. Despite some speculation, officials also said there were "no illegal marijuana grow sites nearby, so that also was excluded as a possible cause."
Fresno County officials said the cost of fighting and extinguishing the fire was $193 million. The cost of recovery and rebuilding is estimated to exceed $500 million.
"We all want to know a definitive cause to what started the devastating Creek Fire last year," Dean Gould, forest supervisor of the Sierra National Forest, said in a statement. "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."
Gould said that at this point, "the case is considered closed, pending further leads."
The fire began September 4 and was finally declared contained just before Christmas. The blaze destroyed 853 structures and damaged 64, according to the California Department of Forestry and Fire Protection (Cal Fire).
Twenty-nine fire fighters were injured fighting the blaze, Cal Fire said
Three fire complexes -- where two or more fires merge -- have burned more acreage than the Creek fire, including last year's August Complex, which charred 1 million acres.