SAN FRANCISCO — 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 said.

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

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

In eight fires there was "evidence of alleged violations of state law" by the utility 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-chair 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," McCallum said.

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 was spent fighting fires and on recovery north of San Francisco in October, including debris removal and infrastructure repair

The destruction prompted $10 billion in insurance claims.

Hundreds of homeowners and relatives of those killed have sued PG&E, which has sought to raise rates to cover possible judgments.

PG&E said in a statement that the company believes its "overall programs met our state's high standards" for maintaining electrical equipment. The utility said it inspects its 2 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, the utility 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," said state Sen. Jerry Hill, a Redwood City Democrat, a longtime critic of the utility.

In March, PG&E announced it would start switching off power to minimize sparks in vulnerable areas during times of extreme fire danger. PG&E and some other state utilities previously have 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 fall, investigators said Potter Valley experienced wind speeds up to 67 mph, 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 (16 kilometers), 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 fire.

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," Bettger said.

Sen. 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 fires 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.