An investigation by the California Department of Forestry and Fire Protection (CalFire) has concluded that four 2017 wildfires in Northern California were caused by trees coming into contact with power lines. CalFire has also found evidence that utility Pacific Gas & Electric (PG&E) violated state vegetation-management laws, potentially causing three of those four fires.

The so-called “October Fire Siege” included more than 170 fires and affected more than 245,000 acres, according to CalFire. The four fires discussed in this investigation occurred in Butte and Nevada counties.

CalFire determined PG&E did not cause the La Porte Fire, which was the most damaging of the four. CalFire stated that tree branches fell onto the utility’s power lines, starting a fire that burned a total of 8,417 acres. Fortunately, no injuries were reported.

However, CalFire found PG&E to be at fault in the McCourtney, Lobo and Honey fires, which burned a total of 973 acres and destroyed 60 structures. In all three cases, CalFire found evidence that the utility violated the state’s Public Resources Code Section 4293, which requires adequate clearance between trees and power lines. In the case of the McCourtney Fire, CalFire stated that PG&E may have failed to remove a tree from the proximity of a power line, which then fell and started the fire.

“The McCourtney, Lobo, [and] Honey investigations have been referred to the appropriate country District Attorney’s offices for review,” according to Cal Fire.

For PG&E, these findings may lead to lawsuits. In fact, the total liability for the utility following all 2017 fires could exceed $10 billion.

In a statement, PG&E defended its vegetation-management programs.

“Based on the information we have so far, we believe our overall programs met our state's high standards,” the statement said. “Under PG&E's industry-leading Vegetation Management Program, we inspect and monitor every PG&E overhead electric transmission and distribution line each year, with some locations patrolled multiple times. We also prune or remove approximately 1.4 million trees annually.”

The statement also calls California’s current fire liability policies “unsustainable,” stating they could impact the utility’s ability to operate in the future. California Gov. Jerry Brown has come out in support of a plan that would change the state’s fire liability rules, which could reduce PG&E’s risk.