Schools were closed and residents were ordered to evacuate as an uncontrolled wildfire in the foothills of the San Bernardino Mountains continued to grow quickly, filling the sky with smoke and threatening communities in Southern California on Monday.

More than 11,000 people were ordered to evacuate as the fire, known as the Line fire, grew, putting more than 36,000 homes, businesses and other structures at risk as it quadrupled in size over the weekend. As of Monday, the blaze had burned through more than 20,500 acres in San Bernardino County since it started on Thursday, according to the California Department of Forestry and Fire Protection, also known as Cal Fire.

The San Bernardino County Superintendent of Schools closed the school and college districts of Bear Valley, Redlands, Rim of the World, Yucaipa-Calimesa and San Bernardino Community College, with the possibility of more closures after conditions are assessed. As of early on Monday, the fire was 3 percent contained.

One school in the city of San Bernardino was closed — Bonnie Oehl Elementary, which was within the evacuation zone.

Three firefighters have been injured. The cause of the fire is under investigation, the authorities said. San Bernardino County, which covers 20,000 square miles, is the largest by area in California, and home to a population of more than two million people.

In Highland, Calif., which is about 70 miles east of Los Angeles and the most densely populated community affected by the fire, ash fell from the sky on Sunday and smoke shrouded the nearby mountains. On Monday, the poor air quality and evacuation warnings in East Highlands and other areas of the city prompted school closures, the Redlands Unified School District said.

A heat wave combined with low humidity and high winds in some areas is creating conditions for extreme fires, officials said. Above-average temperatures were expected in Southern California at least through Monday, the National Weather Service said, while power outages have dogged communities. Temperatures could reach in the upper 90s and triple digits, with elevated to critical fire risk, the Weather Service said.

Over the weekend, the National Weather Service issued a red flag warning for the conditions. The alert covered the region stretching from the Pacific coast to the mountains northwest of Los Angeles.

As the Line fire grew, Gov. Gavin Newsom on Saturday announced a state of emergency for the county, a measure that frees up access to unemployment benefits and waives fees for records and licenses.