Fire officials declined to comment on Edison's statement because no official cause of the blaze has been determined. A multi-agency investigation is continuing.
Investigators were looking at "every possibility" ranging from weather to human or even animal factors, and the nearly yearlong probe probably won't be completed for at least another 30 days, Ventura County fire Capt. Stan Ziegler said.
The Thomas Fire was the second-largest in California history, scorching 440 square miles and destroying more than 1,000 buildings in Ventura and Santa Barbara counties. Two people died.
A month later, heavy rains fell on hills left bare by the fire, unleashing mudslides that killed 21 and left two missing.
Edison's disclosure came as an update to investors but was released publicly to keep communities and customers informed, the company said.
Robert Curtis, an attorney representing 450 victims of the fire and resulting mudslides, said he believed that Edison made the admission because witnesses already have come forward to say they believe the utility's equipment was involved.
"They did it because they're faced with insurmountable evidence that it's their responsibility," he said.
Curtis' firm is one of several representing thousands of fire and mudslide victims in a consolidated suit that will be heard in Los Angeles.
The announcement came weeks after Gov. Jerry Brown signed legislation allowing utilities to bill customers to pay for future legal settlements due to 2017 wildfires.
Follow Christopher Weber at https://twitter.com/WeberCM.
Associated Press journalist Robert Jablon in Los Angeles contributed to this report.