The deadly accident happened as police responded to reports that children were driving an automobile and a mother who said her children might have been kidnapped, KSAZ-TV, Phoenix, reported Thursday.
Officers found the car and tried to pull it over, but the vehicle veered to the right and hit a pole, said police Sgt. Steve Martos.
The boy was found at the wheel of the car. He had minor injuries.
The sister was found inside the vehicle. She was taken to a hospital where she died.
The children's mother told police she put them to bed about 9:30 p.m. When she found them missing, she thought they had been kidnapped.
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