A California Highway Patrol spokesman said the vehicle had been rented by a Los Angeles church group. The van was headed for San Francisco when it crashed about 11:00 pm PT, reports CBS affiliate KCOY-TV in Santa Maria.
The identity of the church was not released and investigators said they did not immediately know the cause of the crash.
According to police, none of the passengers, who ranged in age from 5 to 60, was wearing a seat belt. A man and a woman were in critical condition, a second man was in stable condition and three adults were treated for minor injuries and released at different hospitals. Aretha Fike, 5, was in serious condition at Santa Barbara Cottage Hospital, nursing supervisor Carol Dolyak said.
The accident occurred in Atascadero, about 210 miles north of Los Angeles.
The van apparently slid broadside across the northbound side of the highway before overturning and tumbling across the southbound lanes. No other vehicles were involved.
The crumpled vehicle came to rest on its passenger side in a 10-foot-deep roadside ditch. Rescue workers righted the vehicle by propping it with boards to gain access to those trapped in the wreckage.
Purses, backpacks and plastic utensils were strewn on the highway, surrounding a trail of debris left by the van as it tumbled across the center divider and two lanes.
Investigators said they were puzzled by the accident, which occurred on a relatively straight section of the highway. The driver was among the dead.
"It's kind of bizarre. It doesn't make any sense," CHP Sgt. Christopher Day said. "The road is straight. There is no unusual slope or grade. It's like the driver lost control."