The agency said in a report Friday that investigators determined the fires "were caused by electric power and distribution lines, conductors and the failure of power poles."
All involved equipment owned by San Francisco-based Pacific Gas & Electric. A message left with PG&E was not immediately returned Friday afternoon.
It said the cluster of fires burned at least 245,000 acres in Northern California. About 11,000 firefighters from 17 states and Australia helped battle the blazes.
The deadly fires in Friday's report burned in Mendocino and Napa counties.
CalFire investigators are still probing other fires in October and December.