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A multivehicle traffic collision on the 15 Freeway in the Cajon Pass preceded a brush fire that spread to more than 20 acres, causing hours of traffic backup and leaving a CHP officer and firefighter injured on Saturday.

The San Bernardino County Fire Department responded to the blaze in a canyon near the northbound freeway, south of State Route 138, the Fire Department tweeted around 1 p.m. San Bernardino National Forest firefighters also responded.

The Scales Fire — named for its proximity to nearby truck scales — began as a result of an incident on the freeway, the Forest Service tweeted, referring media to the California Highway Patrol.

CHP spokesman Officer Brian Alvarez said on scene that a motorcyclist struck a tour bus and two other cars and was thrown from the motorcycle. A responding CHP officer and firefighter were helping the motorcyclist when, for unknown reasons, a flash ignited, burning both of the first responders, Alvarez said.

The San Bernardino Sun reported that an oxygen tank caught fire and likely caused the nearby brush to ignite, but Alvarez said fire officials were still investigating.

The injured CHP officer suffered injuries to her hands and face, and was hospitalized, Alvarez said; it was not clear if the firefighter or motorcyclist were seriously hurt.

As the fire rapidly spread, lane closures were put in place and traffic backed up. Three lanes on the northbound 15 Freeway were shut down, Caltrans said on Twitter.

The fire moved at a “rapid rate” and was estimated to have burned 10 acres, the Forest Service tweeted around 1:26 p.m. About 20 minutes later, the Forest Service tweeted that firefighters were making “good progress.”

By 8 p.m., 20 to 25 acres had burned and the blaze was 60 percent contained, the county Fire Department stated on Twitter. About 350 personnel were on scene, according to the tweet.

Air tankers responded, dropping bright retardant on flames, photos from the scene showed. It was not clear by late evening if the fire was completely contained.

Lanes on the 15 Freeway reopened by 10 p.m., and traffic was moving again. Some motorists sat on the pass for hours, and vehicles overheated, drivers told KTLA.

The No. 2 lane of the westbound 138 remained closed late Saturday.

The blaze broke out about two weeks after the 3,500-acre North Fire burned nearby, off the 15 Freeway, north of the 138.

The July 17 fire spread through the Cajon Pass, damaging several homes and vehicles after jumping the freeway.