Crews planned heavy air attacks Thursday at daybreak to halt the spread of a Southern California wildfire that has chewed through more than 25 square miles of bone-dry brush, threatening dozens of homes as well as a multimillion-dollar agriculture industry and valuable oil production fields.

Crews also kept an eye on five big electrical transmission lines that supply Ventura and Santa Barbara with electricity.

A red flag warning of extreme fire danger remained in effect because of hot, dry weather but gusty Santa Ana winds were expected to ease. Temperatures were expected to top 100 degrees.

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The wind was a crucial factor in fighting the blaze as it burned through grass and light brush, Ventura County fire spokesman Bill Nash said.

"It burns very quickly, almost like a fuse," he said. "It'll burn just as fast as the wind can push it."

Firefighters focused Thursday on the fire's eastern and western flanks just north of Moorpark, about 40 miles northwest of downtown Los Angeles.

The 16,400-acre blaze was 40 percent contained and moving away from populated areas, Nash said.

It was expected to be contained on Saturday.

Mandatory evacuation orders for some areas were lifted Wednesday night.

Seventy-five homes remained threatened, down from 1,000 homes a day earlier, Nash said.

"We had a pretty good night. The fire did flare up a little bit on the east end. It didn't grow much," Nash said.

"We think we will lose the strong east winds," Nash said. "We are a little hopeful. If the weather cooperates ... we think today could be a really good day for us."

More than 1,800 firefighters were on the line, along with 21 bulldozers. Twenty-one helicopters were expected to renew the aerial attack. Nash said firefighting costs had topped $3 million.

On Wednesday, the helicopters dumped water on flames moving toward orchards. Ground crews doused burning shrubbery at the edges of avocado and citrus groves. County fire Capt. Ron Oatman said the fire likely burned some trees but could not confirm any orchard damage.

"A lot of the ranchers would rather see their homes burn down than their orchards, because that's their livelihood," he said.

Firefighters were also concerned about a 36-inch natural gas pipeline and oil production fields at the top of a ridge, Oatman said.

The blaze was the largest of several that erupted in Southern California Tuesday as Santa Ana winds blew in from the northeast, pushing back the normal flow of cool and moist ocean air.

Fire officials said the blaze began near an agricultural mulch pile, but the cause remained under investigation. The Sheriff's Department earlier said it was apparently caused by spontaneous combustion in a pile of manure at a ranch. When manure breaks down it emits volatile gases which, when combined with the late summer heat and sunshine, can explode and catch fire.

Two outbuildings were destroyed but no homes had been damaged.

In San Bernardino County, a 16-year-old boy was arrested for investigation of starting a 450-acre wildfire Wednesday afternoon near Yucaipa in San Bernardino County. The fire was 40 percent contained.

The teenager was spotted on a bicycle where the blaze started, said Bill Peters, a spokesman for the California Department of Forestry and Fire Protection.

The hot, dry winds also increased fire activity in the San Gabriel Mountains, where remnants of a month-old, 251-square-mile wildfire still smolder. The blaze destroyed 89 homes and caused two firefighter deaths in August.

Completion of containment lines around the giant blaze was expected Thursday, "but we'll be here for a long time after that," Forest Service fire spokesman Bob Poole said.

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