What caused California’s lethal wildfires?

Questions are being raised about the cause of the deadliest week of wildfires in California’s history - and whether authorities did enough to warn vulnerable residents as the flames edged closer to populated areas.

At least 31 people have died in the wildfires, which erupted on Sunday night. Hundreds more are missing and thousands are homeless.

The cause of the blazes - the biggest of which are in Northern California’s Sonoma, Napa and Mendocino counties - remains unclear.

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Mike Mohler, a Cal Fire battalion chief, said investigators are looking into reports of downed power lines to determine whether they caused some of the wildfires, which now number almost two dozen.

High-speed winds may have helped spread the fires, says CNN. Hurricane-force gusts have been reported in Sonoma County.

The dry conditions and proliferation of dried vegetation are also thought to have contributed to an almost “perfect storm” of factors that have fanned the infernos.

The three largest fires started between 9pm and 11pm on Sunday, “ripping into neighborhoods when many residents had started to go to bed”, says CNN.

There have been claims that authorities failed to give residents adequate warning.

In Sonoma County, police called residents’ landlines advising them to evacuate, and sent alerts through a voluntary text-message system. But only about 10,500 of the county’s 500,000 residents had signed up for alerts as of June, says The Washington Post.

More dry and gusty winds have been forecast, threatening to further fan the flames over the weekend. Chris Canning, the mayor of Calistoga, in the Napa Valley, told residents and visitors to evacuate and stay away, ABC News reports.

“Your choice to stay, and there have been very few of them, is a distraction to our first responders,” Canning said. “You will not be given life safety support at this point. You are on your own.”