MIDDLETOWN — A homeowner’s faulty outdoor wiring for a hot tub sparked the massive Valley fire last year that burned more than 76,000 acres in southern Lake County, killing at least four residents, burning four firefighters and destroying 1,280 homes, Cal Fire investigators have determined.

The wiring, which was not up to code, had been installed by the Cobb Mountain homeowner and was touching dry grass and leaves, investigators found. The debris caught fire on the hot afternoon of Sept. 12 and flames quickly spread, growing to 400 acres within 90 minutes, overrunning a team of firefighters and torching homes.

The investigative report unveiled Wednesday by Cal Fire officials marks the state’s first official verdict on the cause of the blaze, which at its height, displaced roughly 20,000 residents and drew a huge response of up to 4,000 firefighters. When it was fully contained Oct. 15, the Valley fire had become the third-most destructive wildfire in California history, with estimated insured losses and damage to government infrastructure alone totaling $1.5 billion. Firefighting costs totaled $57 million.

It was “a tragedy that’s affected everyone in Lake County,” said Sheriff Brian Martin, one of several officials who spoke during a press conference at Cal Fire’s Middletown station, where officials announced their findings.