Cal Fire said Friday its investigators were unable to determine what caused the destructive Glass Fire to spark in the North Bay last fall.
"After a very meticulous and thorough investigation, which included months of follow-up on information provided by the public, not enough evidence was available to conclusively identify the cause," Cal Fire said in a statement.
The Glass Fire broke out at 3:48 a.m. on Sept. 27, according to Cal Fire. It went on to burn over 67,000 acres and destroy 1,555 structures in Sonoma and Napa counties before crews were able to reach full containment on Oct. 20.
Cal Fire will keep the cause of the blaze as undetermined unless conclusive evidence pointing to a determination is found.
Cal Fire said dry brush, gusty winds, low humidity and warm temperatures led to "extreme rates of fire spread," making it difficult for investigators to determine a cause.