The California Public Utilities Commission said the October fires were started because San Diego Gas & Electric Co. and Cox Communications violated state regulations regarding the maintenance of power lines.
Two of the fires started when utility wires touched in strong winds, the commission said in a report released Tuesday. A third started when a tree limb fell onto one of the utility's power lines, the report said.
Two of the October fires merged to scorch more than 307 square miles, destroying 1,141 homes, killing two people and injuring more than 40 firefighters. The Witch Creek Fire was the largest of five major fires that ravaged San Diego County last fall, charring some 2,000 residences, causing hundreds of thousands to flee their homes and killing nine people.
The fire allegedly started by a tree limb destroyed 206 homes and burned more than 14 square miles.
The commission accused the utility of failing to cooperate with investigators who were sent to probe the wildfires, hindering the release of a more timely report.
The utility said regulators lack the evidence to support their claims. The utility also denied blocking access to its staff, saying employees were busy making repairs and re-establishing power when regulators first requested interviews.
The state report "is full of speculation and faulty conclusions, with sparse evidence if any to support its claims," the utility said in a statement.
The commission said one of the fires started when a broken Cox "lashing wire" used to bind other cables made contact with the utility company's line. Cox officials said their fiber optic cables do not carry electrical current that would start a fire, and were intact prior to the extreme Santa Ana winds.
"Cox has cooperated fully with all agencies during this process," the cable company said in a statement. "Staff statements in the report are inconsistent with the facts."
San Diego City Attorney Michael Aguirre said he plans to add Cox Communications to the city's lawsuit to recover $40 million in firefighting costs and damage to city property from SDG&E.
At a news conference Tuesday outside Sempra Energy, SDG&E's parent company, Aguirre said the report proves the wildfires wouldn't have happened if the utilities had maintained their equipment.
"We can't do anything about the loss of lives. We can't do anything about the loss of property now except try to get compensation, but what we can do is take action to make sure this doesn't happen again," Aguirre said.
The utility is also fighting lawsuits from more than 300 fire victims.