Firefighters are making progress on a wildfire that swept across a Southern California freeway Friday, destroying 20 vehicles and sending motorists running to safety before burning four structures, authorities say.

The fire started at about 2:33 p.m. in the Cajon Pass along Interstate 15 – the main highway connecting Southern California and Las Vegas – and quickly spread to 3,500 acres. Embers and other materials caused cars to catch on fire.

U.S. Forest Service spokesman Lee Beyer says hundreds of firefighters, aided by water-dropping aircraft, have contained 5 percent of the blaze. Uriah Hernandez, another spokesman for the agency, said no injuries had been confirmed, but MyFoxLA reported at least five were injured by the blaze.

Strong winds spread the fire to the rural community of Baldy Mesa, where it burned at least five houses and threatens about 50 more. Some areas were evacuated in the Cajon Pass area.

Burnt-out cars sat on the highway. and other cars and trucks were at a standstill. Traffic was backed up on the freeway as a plane dumped fire retardant on the fire that swept up a hillside.

California Highway Patrol spokesman Steve Carapia said 50 to 75 vehicles were left abandoned on the freeway.

MyFoxLA reported that firefighting efforts were initially grounded due to drone activity.

Raquel Martinez, 34, was traveling to Las Vegas with her husband for the weekend when they got stuck in northbound traffic on the I-15 for about an hour.

The sky darkened to black. As they drove by, cars were covered in "pink powder" -- or fire retardant. Cars meanwhile were being redirected up narrow twisty emergency lanes from the southbound side headed north.

"I haven't seen a fire that big and so close to us. It really was huge," Martinez told The Associated Press.

The drought-stricken California is increasingly prone to wildfires. Some have broken out near freeways, but it’s very unusual to have vehicles caught in the flames.

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The Associated Press contributed to this report.