The $13 million lawsuit was filed Friday in Superior Court by the California Department of Forestry and Fire Protection.
The suit accuses the companies of negligence and seeks unspecified damages to recover money spent fighting the fires. Neither company has commented on the lawsuit.
The California Public Utilities Commission has said that Cox and the utility violated state regulations regarding the maintenance of power lines.
State regulators are blaming them for downed power lines that started the fires which killed two people and destroyed 1,300 homes.