Many vehicles, including this one, were destroyed by the Valley Fire in Lake County, Calif. Credit: Randy Pench/The Sacramento Bee via AP

The lethal Valley Fire that destroyed nearly 2,000 buildings and killed at least four people in  September was started by faulty wiring at a home outside the community of Cobb in Lake County, California, an investigation found.

The fire, which raced through Cobb, Middletown and several other communities in a matter of hours, charred more than 76,000 acres. It destroyed 1,280 homes, 27 apartment buildings and 651 other buildings, making it the third most destructive blaze in state history.

Four people were confirmed dead in the fire. A fifth person, who has been missing since the fire started, has not been found. Four firefighters sustained second-degree burns. The fire cost an estimated $56 million to suppress.

Investigators traced the cause of the fire to an electrical connection for a hot tub at a home on High Valley Road, according to a report released Wednesday by the state Department of Forestry and Fire Protection, or CAL FIRE.

The copper wires were heated to the point at which they melted – at least 1,981 degrees. Sparks arcing from the connection also might have ignited dry grass surrounding the wiring.

Destroyed homes and vehicles scorched by the Valley fire line Jefferson St. in Middletown, Calif, on Monday, Sept. 21, 2015. According to fire officials, the blaze ranks as the sixth worst wildfire in California history after destroying more than 1,000 structures, burning more than 75,000 acres and leaving at least three people dead.Credit: AP Photo/Noah Berger

The homeowner told investigators that he had not mowed weeds and grass for nearly a month prior to the fire.

The connection, which was installed by the homeowner, did not meet code standards, and no permits or plans for the wiring were on file with the county building division, according to investigators.

The report has been sent to the Lake County district attorney’s office for review, according to CAL FIRE spokesman Daniel Berlant.

Check out Reveal’s previous coverage of the Valley Fire and two other Lake County fires last year.

Eric Sagara can be reached at Follow him on Twitter: @esagara.

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Eric Sagara is a senior data reporter for Reveal. He joined Reveal following a news applications fellowship at ProPublica, where he worked on projects about pharmaceutical payments to doctors, deadly force in police agencies and the trail of guns in the United States. Prior to that, he was a reporter on The Newark Star-Ledger's data team. Sagara is originally from Arizona, where he reported on business, education, crime, wildfires and government. He is based in Reveal's Emeryville, California, office.