"They've determined that the Valley Fire was caused by faulty residential connection," said California Department of Forestry and Fire Protection Director Ken Pimlott.
The Valley Fire sparked on Sept. 12, 2015. It killed four people, severely injured four firefighters and burned more than 76,000 acres.
It destroyed more than 1,300 homes and is the third-most destructive wildfire in California history.
"Suppressing the fire was just the beginning. This community has really banded together to move forward, and the recovery process is slow. It's painful, and it's difficult," Pimlott said.
The fire was ignited by faulty wiring on a hot tub. The homeowner did not have the proper permits, and the wiring did not meet building codes. Investigators determined "there were no other plausible causes for this fire."
"It's extremely important to note the strength and resilience of this community that did lose so much," said Supervisor Rob Brown.
The district attorney's office is reviewing the final report to determine if any charges are appropriate.
"This will go forward. Again, whether it's civil, whether it's criminal, those questions are all being asked," Pimlott explained.
This video includes clips from KGO-TV, KRON, KPIX and KTVU and images from the California Department of Forestry and Fire Protection.