The official cause of the Glass Fire in Napa and Sonoma counties last fall will remain undetermined pending the revelation of more conclusive evidence, and Cal Fire investigators say they have not found enough to make a determination.

On Friday, Cal Fire's investigative team revealed that, following nearly 11 months of investigating, they were unable to say what sparked the fire that ultimately triggered dramatic evacuations and destroyed over 1,500 structures.

"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," the agency said in a statement, per the Santa Rosa Press Democrat.

The fire broke out at 3:48 a.m. on September 27, and by the following evening it had spread from Napa into Sonoma County, leading to the mass evacuation of the Oakmont senior community, the Skyhawk neighborhood in Santa Rosa, and parts of Kenwood and Glen Ellen as well. The fire ultimately burned 67,000 acres over the next several days and destroyed 640 homes between the two counties. It also damaged or destroyed 20 wineries, leading it to be called the worst wildfire in Napa County history.

The official origin of the fire remains the 200 block of North Fork Crystal Springs Road — a narrow lane off of the Silverado Trail, north of St. Helena. Investigators were already focused on this area, and a winery property there, in the days immediately following the fire, but they say, multiple factors made it difficult to pinpoint a source for the blaze.

"Tinder-dry vegetation and strong winds combined with low humidity and warm temperatures contributed to extreme rates of fire spread which contributed to the difficulty in determining the cause of the fire," Cal Fire said Friday.

This fire is fairly unique among recent major wildfires in the region, because from the start it did not appear that PG&E transmission lines or equipment were to blame. Sonoma County has already filed criminal charges against PG&E over the Kincade Fire, which broke out a year earlier, in October 2019 — and this was after investigators determined the fire's origin was a malfunctioning transmission tower near Geyserville.

Sonoma and Napa counties, along with much of the rest of the Bay Area, were still dealing with the effects and ongoing smoldering of several lightning complex fires last September when the Glass Fire broke out. Those other, larger fires began with lightning storms on August 17.

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