Fire managers began releasing engines and air power from a fire Saturday that destroyed at least 15 houses as crews secured containment lines near buildings and made progress in the Sierra backcountry to keep the flames out of the Lake Tahoe basin.

Meanwhile, officials broadened their investigation of the fire's cause. They initially said they suspected the blaze was started by teenagers in Kings Canyon the day before the fire erupted Wednesday, but they said Saturday that the fire could have started last weekend and smoldered undetected for days.

The wind-driven blaze, which scorched nearly 7,600 acres, also destroyed a business and 25 outbuildings. It was 50 percent contained Saturday, and no longer posed an imminent threat to communities in northwest Carson City or surrounding areas in Washoe Valley, officials said.

Fire officials said the blaze could be fully contained by Tuesday with good weather. Nearly 2,000 firefighters remained on the lines Saturday, assisted by more than 120 engines and water tenders, bulldozers and aircraft that included three heavy air tankers.

In California, hundreds of people were forced to evacuate Saturday when a wildfire broke out in northern Los Angeles County. The 2,100-acre blaze threatened Sand Canyon, a community of more than 100 houses, Placerita Canyon and the Placerita Canyon Nature Center.

About 750 firefighters were on the scene, battling the blaze with water-dropping helicopters. No homes or structures had been destroyed, officials said.

Another wildfire in the area shifted away from two rural communities, allowing hundreds of residents to return to their homes. The fire in Lake Hughes had blackened 15,988 acres but was about 54 percent contained, officials said.

The blazes were among dozens of wildfires in California that burned more than 31,000 acres during the week. Karen Terrill of the California Department of Forestry and Fire Protection said her department found 200 fires in 48 hours.

A firefighter sets backfires Saturday near Route 14 in Santa Clarita, Calif.