HEMET, Calif. (AP) — Firefighters struggled Thursday to gain control of major California wildfires that have grown explosively and forced extensive evacuations amid a searing heat wave.

The deadly and destructive Fairview Fire in Southern California expanded in two directions on Wednesday, covering more than 30 square miles of Riverside County. It was just 5% contained.

In the Sierra Nevada, the Mosquito Fire had scorched nearly 9 square miles, forcing evacuations in Placer and El Dorado counties.

“As you’ve seen with the smoke column coming up, this fire continues to give us a hard time,” Cal Fire Division Chief Mike Rufenacht said in a video briefing.

Another dangerous blaze burned near the Big Bear Lake resort region in the San Bernardino Mountains east of Los Angeles. It was just 2% contained after scorching nearly 2 square miles.

The prolonged heat wave was expected to abruptly end, in Southern California at least, by the weekend as remnants of the current Hurricane Kay arrive, bringing rain. Kay was off southern Baja California early Thursday but some showers and thunderstorms associated with the hurricane were already reaching Southern California.

The Fairview Fire erupted Monday amid triple-digit heat and spread swiftly, killing two people who were found in a vehicle, severely burning another person while destroying seven structures and damaging several others.

The cause of the fire was under investigation. Southern California Edison notified the California Public Utilities Commission that “circuit activity” occurred close in time to when the fire was reported, the Los Angeles Times said. The activity was not specified.

The Mosquito Fire burned several structures and at least 10 cars near the Gold Rush-era community of Michigan Bluff about an hour northwest of Sacramento.

Near the Oregon border, the Mountain Fire covered more than 18 square miles of rural Siskiyou County and was 30% contained. It began Sept. 2.

Meanwhile, a wood products company said Wednesday that it is investigating whether a fire that killed two people as it swept through the Northern California town of Weed was caused by the possible failure of a water-spraying machine used to cool ash at its veneer mill.

Roseburg Forest Products Co. also announced that although the investigation was not complete, it was planning to provide up to $50 million for a community restoration fund.

The Mill Fire erupted Sept. 2 at the company’s facility in Weed on Interstate 5, about 280 miles northeast of San Francisco.

Roseburg Forest Products said in a news release that its mill produces its own electricity in a co-generation facility fueled by wood remnants, and the ash that is ejected is sprayed with cooling water by a “third-party-supplied machine.”

“Roseburg is investigating whether the third-party machine failed to cool the ash sufficiently which thereby ignited the fire,” the release said.

Hundreds of people fled Weed as the fire spread, destroying 107 structures and damaging 26 others. The blaze eventually grew to more than 6 square miles. The fire was 65% contained Wednesday, with minimal activity.

Roseburg said its fund will assist residents with temporary shelter, medical supplies and treatment, transportation, clothing, food and water, and child care services.

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