(KRON) — The cause of the deadly Valley Fire in Lake and Napa counties was a faulty residential electrical connection.

The massive wildfire that start on September 12, 2015, destroyed more than 1,300 homes and killed four people in Lake County.

From CAL FIRE:

After a comprehensive investigation, CAL FIRE has determined that the 2015 Valley Fire was caused by a faulty residential electrical connection. Investigators were dispatched as part of the initial response to the fire and immediately began working to determine the origin and cause of the fire.  The ensuing investigation uncovered evidence that a residential electrical connection arced and ignited surrounding dry grass.

The Valley Fire burned 76,067 acres, took the lives of four civilians, seriously injured four firefighters, and destroyed 1,955 structures. At its peak, over 4,000 firefighters battled the blaze which cost nearly $57 million to extinguish. The fire became the 3rd most destructive wildfire in California’s recorded history.