MIDDLETOWN (CBS SF) – Nearly one year after the Valley Fire tore through Lake County, investigators said the massive blaze was sparked by faulty electrical wiring in a home.

The fire, which broke out on September 12, 2015, became the third most destructive fire in California history.

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According to a statement from Cal Fire (.pdf), a residential electrical connection arced, igniting dry grass nearby.

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Four people were killed, four firefighters were seriously injured and 1,955 structures were destroyed in the Valley Fire. A total of 76,067 acres were burned (118 square miles,) an area more than two and a half times the size of San Francisco.

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According to The Associated Press, the Valley Fire caused at least $700 million in insured damages. Cal Fire said the cost of extinguishing the blaze was $57 million.