The fire began on July 27 and was spurred on Monday by an ominous high-pressure system that brought hotter, drier and windier weather to the area, according to the California Department of Forestry and Fire Protection, or Cal Fire.
The fire has destroyed dozens of homes and other buildings, and more than 11,000 structures remained threatened, the department said.
Mandatory evacuations remained in effect across Mendocino, Lake and nearby Colusa counties, though some people were allowed to return home on Monday afternoon, Cal Fire said.
Fire in Mendocino Complex the fifth largest in California history
Nearly 4,000 fire personnel, including 441 fire fighters, were battling the wildfire.
It was unclear what caused the blaze, but the fire was one of more than a dozen burning amid record-setting heat waves in the drought-stricken state, according to Cal Fire. Tens of thousands of residents across California have been displaced by wildfires this season, Gov. Jerry Brown told reporters Saturday.
President Donald Trump declared a state of emergency in California on July 27, authorizing the Federal Emergency Management Agency to help firefighters working to respond to the disaster areas
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