Firefighters walk in a neighbourhood destroyed by the Tubbs Fire in Santa Rosa, California, in October 2017. File picture: Stephen Lam/Reuters

Los Angeles - Investigators have concluded that private utility lines, not ones owned by utility giant Pacific Gas & Electric, started a massive fire in Santa Rosa that killed 22 people and burned thousands of homes in 2017.

In a statement, the California Department of Forestry and Fire Prevention said the Tubbs fire was caused by "a private electrical system adjacent to a residential structure."

"Investigators did not identify any violations of state law," the statement said, "related to the cause of this fire."

The long-awaited investigation comes as PG&E is expected to file for bankruptcy because of huge losses related to the Camp fire last year that destroyed more than 90 percent of the town of Paradise. Authorities are still investigating the cause, but it appeared to begin near a PG&E line.

PG&E is also facing massive losses from the wine country fires, of which Tubbs was the worst. Those fires killed more than 40 people.

Early on in the Tubbs investigation, focus turned to electrical equipment that fire investigators seized at 1128 Bennett Lane in Calistoga.

There was speculation the equipment was the cause of the fire and that it belonged to PG&E. Some plaintiff attorneys alleged that PG&E failed to properly trim trees near power lines and that the winds might have sent branches falling onto equipment, sparking the blaze.

But a court filing from PG&E last year indicated the equipment was owned by an unidentified private party, not the utility. Moreover, the filing claimed, it did not appear that PG&E equipment in that area had burned.

Investigators used drones and aerial photography to help put together a comprehensive picture of the fire and gathered evidence on the ground to figure out how it started.

Investigators have already linked PG&E lines to some of the other October fires that hit wine country, including the Atlas fire, which killed six people and destroyed 400 homes, and the Redwood Valley fire, which killed nine and razed 500 structures.