(CNN) -- A 6-year-old girl has died after her 8-year-old brother took her on a joyride in their mother's car and crashed Wednesday night in South Phoenix, police said.
The children's mother believed they had been kidnapped when she discovered they were missing, said Sgt. Steve Martos of the Phoenix Police Department.
"It's a tragic story, all the way round," said Martos, who did not give the children's names.
The mother gave the children a bath and put them to bed around 9:30 p.m., he said.
The children's grandmother, who lives nearby, was driving home when saw her daughter's front door open.
She went in, woke up the woman, and they discovered both the children were missing, Martos said.
As they came out to check with neighbors, the children's mother discovered that her car -- a 2010 Hyundai Elantra which had been parked outside -- was also gone, Martos said.
The frantic mother called 911 around 10:25 p.m., saying she believed someone had kidnapped her children.
Police said they started to get numerous 911 calls starting around 10:15 p.m. One caller said the car had struck a fence. Another who followed the vehicle reported that children were inside.
At around 10:30 p.m., when police tried to pull the vehicle over, it veered to the right and hit a pole, Martos said.
"This was one of those situations when the officers believed they were trying to rescue kids from a possible kidnapper," Martos said.
"When they ran up to the vehicle, they found an 8-year-old in the driver's seat, and a 6-year-old in the passenger seat, severely injured."
Responding officers started life-saving measures until the fire department arrived, Martos said.
Both the children were dressed in their pajamas. The 6-year-old was not wearing a seat belt. The air bag did not deploy.
The mother, who lives within a block of the where the accident occurred walked to the scene and discovered that her children were in the car.
The 6-year-old died around 4 a.m. Thursday in a local hospital. The 8-year-old boy was treated for minor injuries.
Martos said at this point they do not know what prompted the 8-year-old to take his mom's car. They have not spoken to the boy, but no criminal complaints are expected to be filed, he said.
CNN's Meredith Edwards contributed to this story.