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OCEANSIDE, Calif. — Investigators sought Tuesday to determine what caused a late-night solo freeway crash that killed a motorist in the far northern reaches of San Diego County.

The man was behind the wheel of a black Ford Ranger that veered off a curving stretch of northbound Interstate 5 near the Santa Margarita River in the Oceanside area shortly before 10 p.m. Monday, according to the California Highway Patrol.

The truck went off the east side of the roadway, struck a tree and caught fire, CHP public-affairs Officer Mark Latulippe said.

Witnesses rushed over to the burning vehicle and pulled out the mortally injured driver, Latulippe said. Medics tried in vain to revive the victim before pronouncing him dead at the scene of the accident.

The man remained unidentified Tuesday afternoon, Latulippe said.

The crash forced a partial closure of northbound I-5 in the area for about 2 1/2 hours, the CHP reported.