A dozen wildfires that burned thousands of homes in California’s wine country and killed at least 15 people last October were started by Pacific Gas & Electric power lines and utility poles, state fire officials have said.

The California Department of Forestry and Fire Protection released a report on Friday on its inquiry into some of the wind-driven fires that ravaged Mendocino, Humboldt, Butte, Sonoma, Lake and Napa counties.

Falling trees and branches hitting power lines were the most common cause, but one fire erupted after PG&E tried to re-energise a downed line, investigators found.

In eight fires there was “evidence of alleged violations of state law” by the utility company and those cases have been referred to county prosecutors for review, according to the forestry department.

“PG&E has been trying to duck responsibility for the fires, blaming everything from climate change to local fire departments and the state’s liability laws,” Patrick McCallum, co-chairman of a coalition of people affected by the wildfires, said in a statement.

He said Cal Fire’s report “puts the blame where it belongs — squarely on PG&E, confirming it was responsible for many of the fires that devastated so many lives.”

“As victims, we see the report as an important step toward rebuilding and recovery,” he added.

A firefighter tackles a blaze burning along the Highway 29 near Calistoga, California, in October 2017 (Jae C Hong/PA)
A firefighter tackles a blaze burning along the Highway 29 near Calistoga, California, in October 2017 (Jae C Hong/PA)

The dozen blazes were part of the deadliest series of wildfires in California history, which killed 44 people, destroyed 8,800 structures and forced more than 100,000 people to evacuate. About 11,000 firefighters from 17 states and Australia helped battle the blazes.

Nearly 1.5 billion US dollars was spent fighting fires and on recovery north of San Francisco in October, including debris removal and infrastructure repair

The destruction prompted 10 billion dollars of insurance claims.

Hundreds of home-owners and relatives of those killed have sued PG&E, which has sought to raise prices to cover possible judgments.

PG&E said in a statement that the company believes its “overall programmes met our state’s high standards” for maintaining electrical equipment. The utility giant said it inspects its two million power poles regularly and prunes about 1.4 million trees a year.

But “years of drought, extreme heat and 129 million dead trees have created a ‘new normal’ for our state” that has increased the number of large wildfires and the length of the wildfire season, it said.

However, state Senator Jerry Hill, a longtime critic of the company, said: “Climate change and the so-called new normal do not ignite fires. The Cal Fire findings today show that suspected negligence by PG&E did.

PG&E crews work on restoring power lines after a wildfire in Santa Rosa, California, last October (Marcio Jose Sanchez/AP)
PG&E crews work on restoring power lines after a wildfire in Santa Rosa, California, last October (Marcio Jose Sanchez/AP)

In March, PG&E announced it would start switching off power to minimise sparks in vulnerable areas during times of extreme fire danger.

PG&E and some other state utilities have previously resisted such a measure, arguing that cutting off power carries its own risks, including to patients dependent on electrical equipment.

In one fire in Mendocino County last autumn, investigators said Potter Valley experienced wind speeds up to 67mph, causing many tree branches to fall, triggering numerous 911 calls reporting fires, according to Cal Fire’s report.

“An arc from a conductor was witnessed along with the start of a vegetation fire,” the report said. A second fire also was “from an overhead conductor”.

The two sparked a third, merged, and burned 10 miles (16km), the report said.

A responding firefighter said the smoke was blowing sideways and he had to veer around numerous tree branches to get to the blaze.

Another property owner told Fire Captain Specialist Eric Bettger that “he saw a flash to the east and saw the conductors come down”.

“He said the fire crossed the road within seconds,” Mr Bettger added.

A wildfire burns behind a winery in Santa Rosa, California (Jae C Hong/AP)
A wildfire burns behind a winery in Santa Rosa, California (Jae C Hong/AP)

Senator Bill Dodd, a Democrat who represents the Napa area, called the report’s findings “disappointing and deeply concerning”.

He has introduced legislation that would require electric utilities to update wildfire plans to determine when they need to cut power to lines during harsh weather and boost infrastructure.

Cal Fire investigators are still probing other blazes in October and December, including the deadliest blaze in Napa and Sonoma Counties, which PG&E has argued was started by wires belonging to a private homeowner.