As many as 50 vehicles were involved in a chain-reaction accident early Wednesday in fog on a mountain highway, injuring at least 15 people and scattering wreckage for a half-mile, authorities said.

The chain-reaction collision began when two tractor-trailer rigs crashed in northbound lanes, officers said.

Motorist Phil Cokkinos told KTTV's "Good Day LA" program by phone that he saw three separate multi-vehicle accidents as he zigzagged along Interstate 15 near the summit of Cajon Pass. The last crash was the largest, he said.

"There was like three, four big-rigs, a couple cars accordioned in between them. In all between the three accidents, 50 cars-plus," he said.

San Bernardino County fire dispatch supervisor Tom Barnes says 10 patients were being taken to hospitals. Paramedics assessed five others who declined medical treatment.

Cajon Pass rises to an elevation of more than 4,100 feet in the San Bernardino Mountains about 45 miles east of Los Angeles. It carries Interstate 15 traffic between Southern California cities, Mojave Desert cities and Las Vegas.

Northbound traffic stalled at the bottom of the southern side of the pass.

Southern California has been enduring a siege of "June gloom" weather, a deep marine layer that pushes inland from the ocean, bringing low clouds, fog and drizzle.

"It is very, very foggy up there," Lopez said from the highway patrol dispatch center in San Bernardino, south of the pass.