Updated

Firefighters began to get the upper hand early Monday on a 2,334-acre wildfire that had once threatened as many as 400 homes in Riverside County (search).

The blaze, which broke out in a motor home on Sunday afternoon, was 90 percent contained Monday morning, said Stephanie Swanstrom, a county fire information spokeswoman.

It had threatened as many as 400 homes in the Lake Riverside Estates (search) community before firefighters began to surround it overnight. It destroyed two mobile homes and nine vehicles after igniting in a motor home Sunday afternoon on Highway 371 near Foolish Pleasure Road.

"The motor home fire caught nearby vegetation on fire," said Lori Hoffmeister, another county fire information spokeswoman.

As the blaze marched toward Lake Riverside Estates, some 540 firefighters, with help from four helicopters and two air tankers, fought to hold the flames back.

Temperatures, which reached the 90s in the area on Sunday, were expected to soar to near 100 on Monday.

As firefighters worked to completely surround the blaze, investigators were trying to determine what ignited the motor home.

"Apparently it overheated and then caught fire," said Harriet Costo, 74, who watched the blaze from a nearby hill. She said the motor home that burned was "about a city block" from her house on unincorporated county land off Highway 371, five miles from the community of Aguanga (search).

The area is about 100 miles east-southeast of downtown Los Angeles.

Her son, Tom McGiffin, 53, said he watched the blaze quickly jump a road and spread into brush.

"With the wind, it just took off. I'm surprised it got as big as it did," he said.

McGiffin added that it took firefighters some time to arrive.

"That's typical out here because they have to call the main base, notify the fire department, and it's probably 10 miles from the fire station to here. It's just so rural," he said.

The wildfire was the largest in Southern California so far this year. Six months ago, the region was ravaged by a series of firestorms that killed 24 people, destroyed more than 3,600 homes and burned nearly 740,000 acres.