When the lines touched each other, it created an electrical arc.
"The electrical arc deposited hot, burning or molten material onto the ground, in a receptive fuel bed, causing the fire," according to the Ventura County Fire Department. "The common term for this situation is called 'line slap,' and the power line in question is owned by Southern California Edison."
The fire started on Dec. 4, 2017 and burned for almost 40 days in Ventura and Santa Barbara counties, charring more than 281,000 acres, destroying 1,063 buildings and causing deaths of one civilian and one firefighter.
The department's full findings on the fire can be found here.