The suspect was driving a Toyota Sienna minivan that veered off Pope Valley Road and hit a tree shortly before 6 p.m. Sunday, according to the California Highway Patrol.
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The crash, northeast of Calistoga, left six people dead. Their names were not released.
The CHP said the suspect and a passenger who survived the wreck were flown to trauma centers with major injuries and were still hospitalized on Monday. Their conditions weren't released.
The area around the scene of the collision, in the 6100 block of Pope Valley Road was closed until 2:20 a.m. Monday.