The truck driver, Adrian Erazo, 44, who was not injured, was doing a U-turn on the road after parking on the shoulder to sleep, said California Highway Patrol officer Eric Erickson.
But CBS News Correspondent David Dow reports Erazo claims the van did not have its headlights on, even though it was dark. The Highway Patrol is looking into that angle.
The Los Angeles Times reports the driver of the van was not licensed in California and had several traffic violations on his record, including driving without a seat belt and driving without a license.
CBS News Correspondent Bobbi Harley reports the accident happened in California's agricultural hub near Fresno. Police say the van carrying tomato pickers smashed like an accordion on impact.
Workers were sitting in the back of the van without seatbelts, on benches installed along the sides. When the two vehicles collided, they didn't have a chance.
The 1983 beige Dodge Ram van "couldn't avoid hitting the tractor-trailer," Erickson said. "Their brakes locked up."
Brian Yokley of the California Highway Patrol said, "After the impact everyone pretty much moved forward, and as the vehicle stopped due to the impact everyone pretty much continued to move forward inside the van."
He called the accident "one of the worst I have seen."
Dona Portillo's father was among those killed.
She had been working in the fields with her parents, and she and her mother had found a different ride home. "He told my mom, 'You go ahead and go with them, so you can rest and go to sleep,'" Portillo said, sobbing, after returning to the scene.
The injured survivors were taken to University Medical Center in Fresno - a 17-year-old girl in critical condition and a man in serious condition, said Michelle van Valkenburg, a hospital spokeswoman.
California law requires the use of proper seats and seatbelts, but this van was exempt because it was certified as a farm vehicle.
With so many people inside such a small vehicle, it was a recipe for disaster. A tragic accident in the middle of nowhere, with help many miles away.