GENOA, Nev. (AP) _ Thousands of people had to flee upscale homes in the Sierra Nevada after two boys playing with gasoline started a wind-whipped fire that continued to burn out of control on Monday.

Also, lightning-sparked fires burned in Alaska, New Mexico, Utah and Arizona, where 10 Grand Canyon hikers were airlifted to safety.

The Nevada fire 20 miles east of Lake Tahoe had burned about 4,000 acres by Monday evening and threatened about 100 homes near the Heavenly Valley ski resort, about 60 miles south of Reno, before firefighters got help from an incoming storm.

``The situation looks a lot better right now,'' Forest Service spokeswoman Cathleen Thompson said. ``The weather has really helped us. We've had lighter winds, cooler temperatures and higher humidity today. A little rain also helped.''

The fire was only 45 percent contained by Monday evening, but all 3,000 to 4,000 residents evacuated on Sunday were able to return Monday evening _ except for the four families whose homes were destroyed.

``You just have to live with the memories and go on from there,'' said Howard Herz, who lost his $750,000 house in the fire. ``What you have to realize is that what you have as a person, like your integrity, stays with you.''

Thompson predicted that full containment was expected Thursday and the blaze should be out by Saturday.

About 950 firefighters were battling the blaze, aided by air tankers and helicopters dumping retardant. One firefighter was badly burned and seven others suffered minor injuries.

``It could have been much, much worse,'' said local resident Will Kolbe, whose home wasn't damaged. ``Firefighters did a wonderful job saving a lot of homes. We feel very thankful.''

The fire moved into brush and timber after it broke out Sunday afternoon at the base of Kingsbury Grade, 60 miles south of Reno. Within a few hours, the fire had moved toward Heavenly Valley and Genoa, a historic town that's Nevada's first settlement.

``I've lived here all my life and I know how these fires are,'' said Stacy Trivitt. ``I grabbed my dogs, my 9-month-old son, my guns, a photo album and said a prayer.''

``I had to turn out all the horses and all the cattle,'' said Janice Sorenson. ``I could feel the heat and the noise was horrendous. I was running all over the place and the smoke was so black I couldn't see.''

Douglas County Sheriff's Lt. Ross Chichester said the 13-year-old and 14-year-old boys who started the blaze said it was an accident.

In Arizona, wildfires that have ravaged almost 25,000 acres of drought-stricken forests prevented hundreds of people from returning to their homes.

One blaze forced the evacuation by helicopter of 10 Grand Canyon hikers who were heading directly into the fire. The main tourist area on the canyon's South Rim was unaffected.

Wind gusts up to 25 mph fanned the flames of two fires just north of Flagstaff, burning hundreds of acres of forest over the weekend and a handful of homes and cabins, and ravaging habitat for the spotted owls, turkeys and goshawks.

In New Mexico, fire crews were fighting two lightning-sparked fires in the Jemez Mountains about 45 miles northwest of Albuquerque, trying to keep flames away from television towers and a scattering of summer homes.

The larger of the blazes had burned at least 300 acres in a remote area in the Sante Fe National Forest.

Jemez district officials said fire conditions are so extreme that eight out of every nine sparks that land outside the fire line will ignite.

In Utah, a fire burning in the state's west-central desert continued advancing north Monday, prompting officials to draw up evacuation plans for residents in the Eureka area and neighboring towns. About 20,000 acres of forest and grasslands have burned so far.

Meanwhile, in Alaska, the largest of 60 wildfires burning around the state charred more than 27,500 acres in four days, officials said Monday. Firefighters were working to keep it away from Chalkyitsik, a village of 160 about 170 miles northeast of Fairbanks.

``That fire is a gobbler,'' said fire information officer Andy Williams with the Alaska Interagency Fire Center in Fairbanks. Most of the other fires were in remote areas and are not actively being fought, he said.