California investigators have determined that “a private electrical system”—not equipment owned by PG&E Corp.—caused the second-most destructive wildfire in state history, which broke out in wine country in 2017.

The report’s findings come as PG&E prepares to file for bankruptcy as early as next week because of $30 billion worth of potential liabilities stemming from 2017 and 2018 fires.

The Tubbs Fire, which swept through Northern California in October 2017, eventually destroyed thousands of homes and killed 22 people.

California investigators have already named PG&E equipment as the ignition source of 17 of the 2017 fires while alleging violations of state law in 11 of those incidents.

PG&E has said its fire-safety programs have met the state’s high standards and that it has taken a number of steps to reduce fire risk, including pruning or removing about 1.4 million trees a year.

Under California law, utilities including PG&E and Edison International may be held liable for costs if their equipment is found to have caused a fire, even if they followed safety rules.

PG&E said in a federal court filing in December that its equipment may not be responsible for the Tubbs fire. In a report on its potential role, PG&E said it appeared the 2017 blaze was started by “customer-owned equipment” at a residential property in Napa County where there was electrical work that wasn’t permitted.

—With assistance from David R. Baker.

©2019 Bloomberg L.P. All rights reserved. Used with permission