The blaze erupted Thursday and spread to 6 square miles within hours, injuring three people as they escaped from the flames. The burn area was 4,100 acres as of Friday morning.
By Friday evening, one firefighter was injured by a falling tree, according to Cal Fire. The extent of the firefighter's injury was not released.
At least 85 structures were destroyed, according to officials. More than 20 of those were homes at the Rancho Monserate Country Club in Fallbrook.
Of the 10,000 residents who evacuated their homes, about 900 of them were in shelters. According to San Diego County's official Twitter account, 20,000 customers were without power in the area.
The fire reached 15 percent containment but continued to burn along the eastern border of Camp Pendleton. After subsiding overnight Thursday, winds were expected to return Friday.
Some 1,000 firefighters were battling the inferno with help from water-dropping Marine and Navy helicopters, as well as fixed-wing aircraft.
As the flames approached the elite San Luis Rey Downs training facility for thoroughbreds, many of the more than 450 horses were released from their paddocks to prevent them from becoming trapped, said Mac McBride of the Del Mar Thoroughbred Club.
A Facebook video recorded at the scene shows herds of horses galloping past burning palm trees in their chaotic escape.
Horse trainer Scott Hansen said he knows that some of his 30 horses at the facility died.
Gov. Brown declared a state of emergency for San Diego County Thursday, and on Friday President Donald Trump approved an emergency declaration for the state because of all the fires burning in the Southland.
Significant resources also were being deployed to stamp out a new, small fire that began to the east in the Cleveland National Forest near the mountain town of Alpine.
The cause of the two wildfires was not immediately known.
The Associated Press contributed to this report.