The Associated Press

SAN DIEGO — California's state fire agency said Friday that sparking power lines ignited the largest of the wildfires that ravaged Southern California last month. The fire killed two people, burned over 300 square miles and destroyed more than 1,000 homes.

The fire, which blew into the heart of north San Diego from chaparral-covered canyons to the east, merged with a smaller fire also caused by power line sparks, according to the California Department of Forestry and Fire Protection.

State authorities previously blamed a third fire on downed power lines. That fire burned through nearly 15 square miles in a rural area near the community of Fallbrook, destroying 206 homes and damaging avocado groves.

All three fires are in the service area of San Diego Gas & Electric Co., which serves 3.4 million customers in San Diego and southern Orange counties.

The company, a unit of San Diego-based Sempra Energy, says it adhered to regulations in maintaining the low-voltage power lines that caused the fires.

"We believe at the time of the fires our power lines were in compliance with all regulations," said spokeswoman Stephanie Donovan. "When we have the kind of extreme conditions we faced here in San Diego at the end of October, it creates a huge hazard for everybody, including the electrical system."

The state agency said it would not elaborate on the findings until it completes its investigation, said spokeswoman Roxanne Provaznik.

Two families who lost homes in the fires have filed suit in state court against SDG&E, saying the utility failed to clear brush around its power poles and did not insulate power lines to prevent them from sparking. The plaintiffs are seeking class-action certification.

No cause has been determined for another fire that killed eight people, seriously injured four firefighters and a teenage boy, and consumed hundreds of homes in the rural communities along the U.S.-Mexico border east of San Diego.

The fires burned more than 780 square miles acres in Southern California, destroying more than 2,000 homes.