PG&E reported earlier this month that it had detected unspecified "electrical activity" on a line near a Placer County reservoir at about the same time the Mosquito Fire started there September 6.
In its SEC filing, the company said U.S. Forest Service investigators have reached "an initial assessment" that the fire started near one of its power lines. The utility added that, over the weekend, the Forest Service took possession of a PG&E transmission pole and other possible evidence as part of a criminal probe.
In a later statement, PG&E emphasized that the Forest Service has not yet made an official determination about what caused the Mosquito Fire. The service did not immediately respond to a request for comment on its probe, but it often takes investigators many months to reach official determinations about how wildfires started.
"PG&E is cooperating with the USFS investigation," the company's statement said. "While PG&E is conducting our own investigation into the events that led to the fire, we do not have access to the physical evidence that was collected as part of the USFS investigation over the weekend."
The Mosquito Fire has burned about 77,000 acres in Placer and El Dorado counties and destroyed 78 structures, many of them residences. The blaze is now 85% contained.
The fire has resulted in at least one lawsuit against the utility.
A complaint filed last week in San Francisco Superior Court accuses the company of operating its electrical network "recklessly and with conscious disregard for human life and safety" by prioritizing corporate profits over equipment maintenance and managing vegetation along its 25,000 miles of lines in areas identified as at high risk of wildfires.
Fire investigators have found the utility responsible for major conflagrations in six of the past seven years.
Those incidents include the deadliest wildfire in state history, the 2018 Camp Fire, which destroyed the town of Paradise and killed 85 people, and the largest single wildland blaze in state annals, last year's Dixie Fire, which burned nearly 1 million acres in the northern Sierra Nevada.
Those fires have resulted in several criminal prosecutions. PG&E pleaded guilty to 84 counts of involuntary manslaughter in connection with the Camp Fire, which was sparked when a hook on a nearly century-old transmission tower snapped. Earlier this year, it agreed to a civil settlement of charges in the Dixie Fire and the 2019 Kincade Fire in Sonoma County.
The company still faces 31 criminal counts, including involuntary manslaughter, arising from the September 2020 Zogg Fire, which killed four people in rural communities in Shasta County. A preliminary hearing in that case is set for January.
Other fires started by PG&E equipment include the 2015 Butte Fire, which destroyed hundreds of homes and killed two people in Amador and Calaveras counties, and more than a dozen of the fires that swept the northern part of the state in October 2017 — blazes in which thousands of homes were lost and 22 people died.