Story highlights Valley Fire was the third most destructive wildfire recorded in California

It cost nearly $57 million to extinguish

(CNN) A California wildfire that killed four people and cost nearly $57 million to extinguish was ignited by faulty hot tub wiring at a home, officials said.

The so-called Valley Fire raged in Napa and Sonoma counties near San Francisco in September last year.

In addition to the fatalities, including an elderly woman who couldn't get out of her house, four firefighters suffered serious burns that required extensive treatment.

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It left behind a charred trail of destruction: 76,067 acres burned; 1,955 structures destroyed. At its peak, 4,000 firefighters tried to control the roaring flames that threatened to consume more homes.

The fire became the third most destructive wildfire in the state's recorded history, according to the California Department of Forestry and Fire Protection, which conducted the investigation.

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