ALPINE, Calif.—A 7,000-acre wildfire that forced the evacuation of more than a hundred homes near the California-Mexico border may have been caused by an abandoned campfire set by illegal immigrants, authorities said Monday.

The fire had burned nearly 11 square miles of brush and chaparral in the Cleveland National Forest in southern San Diego County.

It appeared to have spread early Saturday from an abandoned campfire set in a side drainage of a canyon, the U.S. Forest Service said in a statement. Evidence at the scene suggested “the campfire was left by undocumented immigrants,” it said.

Forest Service spokeswoman Anabele Cornejo said investigators found food containers and bottles off a park trail.

“Based on collected evidence, we’re making an educated guess that it was probably started by immigrants,” Cornejo said. She did not immediately know whether anyone was detained in connection with the fire.

The wildfire was the latest in a series of blazes around California that have put fire crews to work in triple-digit temperatures.

It prompted sheriff’s deputies to order 125 homes evacuated in the town of Alpine starting Sunday. Residents throughout parts of Pine Valley and Lake Morena, where there are about 1,350 homes combined, were told to remain on standby, said sheriff’s spokeswoman Susan Plese.

More than 1,500 homes and 100 commercial properties could be threatened by the fire, which was about 5 percent contained, but the threat was not immediate, officials said.

A mandatory evacuation remained in effect Monday, but it appeared not all residents fled their homes. A local Red Cross volunteer said only about a dozen families checked into an emergency shelter.

Fire crews worked in blistering temperatures as California’s heat wave lengthened. Five firefighters around the state have suffered heat-related illnesses in recent days, officials said.

“If you get behind on drinking water, you can’t catch up,” said firefighter Jon Sanchioli, 46, who was protecting structures from the forest fire. “We had one guy go down yesterday. We know you’ve got to be careful. If you keep on pushing, your body shuts down.”

Monday’s high in the forest reached 101 degrees, compounded by humidity, according to the National Weather Service. With a 40 percent chance of showers, evening temperatures were expected to drop to the 70s, officials said.

In Joshua Tree National Park _ where a blaze had consumed 1,050-acre acres of dense, desert vegetation _ fire supervisors were asking crews working in temperatures up to 103 degrees to remove their helmets every hour to make sure they were still sweating, fire spokesman Dennis Cross said.

No sweat, he said, could mean a firefighter had “dried up” _ a sign of heat exhaustion.

“It probably feels like it’s 150 up there,” Cross said, adding that crews were drinking about the twice the amount of water and Gatorade they might otherwise consume.

“When you have this humidity and this heat, it really takes a toll on your body,” he said.

The blaze, burning across 1.6 square miles near the Riverside-San Bernardino county line, destroyed a park-owned cabin. It was 67 percent contained.

Farther north, more than 800 firefighters worked to cut lines around an 8,200-acre, or nearly 13-square-mile, fire on ranch land east of San Ardo in southeastern Monterey County.

A lightning strike late Saturday sparked the fire, and erratic winds generated by thunderstorms caused it to spread, officials said.

Off the coast of Los Angeles County, a lightning-sparked fire on Santa Catalina Island was 75 percent contained at 1,094 acres, or 1.7 square miles, fire Inspector Edward Osorio said.

Five firefighters suffered minor, heat-related injuries Monday, with one taken to a hospital for heat exhaustion.

Elsewhere, crews were mopping up a 447-acre blaze in the Cajon Pass that clogged traffic on Interstate 15. The fire started Saturday and was fully contained late Sunday. Two ranches were evacuated and several unoccupied buildings burned.

The cause was under investigation.

Meanwhile, the CDF announced Monday that a 34,217-acre wildfire that burned for more than a week in Northern California was caused by smoking.

The Canyon fire began July 9 in western Stanislaus County and destroyed 11 homes and five outbuildings. No suspects have been identified, officials said.

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On the Net:

National Interagency Fire Center: http://www.nifc.gov