Firefighters battling a wildfire that has burned at least 7,500 acres near a popular southern California resort town are welcoming cooling temperatures and higher humidity Monday.

Crews on the ground are waging an ongoing assault against the "Sheep Fire," burning in the San Gabriel Mountains.

A team of 1,296 firefighters, supported by nine helicopters and 11 fixed wing aircraft have the blaze 20 percent contained. Authorities told FOX News they did not know what caused the wildfire.

The temperature at Wrightwood, at an elevation of about 6,000 feet, was expected to drop below 40 degrees.

Fire crews had spent the previous day cutting fire lines while battling erratic winds, which at times kept air tankers and helicopters grounded.

Officials warned the so-called Sheep fire, which had burned nearly 12 square miles, and still had potential to flare up and the town remained under mandatory evacuation orders.

Of most concern were the winds, which at times gusted up to 50 mph, then dropped to zero.

"It's hard to get a handle on it," Forest Service spokeswoman Barbara Duruisseau said. "The wind could be blowing one way one minute and another way the next."

The fire had destroyed three homes in remote canyons but firefighters had kept it from encroaching into Wrightwood. Between 4,000 to 6,000 residents were ordered to evacuate.

Gov. Arnold Schwarzenegger declared a state of emergency for San Bernardino County, freeing up state resources to battle the fire.

The cause of the blaze was under investigation. It broke out Saturday near Lytle Creek, a small community surrounded by San Bernardino National Forest. Fueled by thick timber and brush, the fire pushed over hills and canyons by fast-moving winds.

Evacuation centers were set up at a high school in nearby Rialto and at the Victorville Fairgrounds.

In Arizona, strong winds kept some residents of the scenic northern Arizona city of Williams from returning to their homes Sunday as crews battled a prescribed burn that grew out of control and threatened part of the community known as the "Gateway to the Grand Canyon."

Punky Moore, a Kaibab National Forest spokeswoman, said the Twin Fire scorched about 1,000 acres, or more than 1 1/2 square miles. It was burning forest undergrowth and ponderosa pines on Bill Williams Mountain.

"We had a little bit of cloud cover and that did help moderate fire activity, but we still had the winds," she said. "That's a concern. We just don't want any fire outside of the lines."

The forecast called for wind gusts as high as 26 mph Monday.

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The Associated Press contributed to this report.