A 6-year-old girl has died from what St. George police believe is carbon monoxide poisoning.

The accident happened at about 1:30 a.m. Thursday, when a mother turned on her car to keep them warm. The car had a broken exhaust system.

"The daughter becomes stiff and goes into a seizure," St. George Police detective Jonathan McInness said Thursday. "The mom panics and is knocking on doors on the apartment complex."

A neighbor called 911 and when police and paramedics responded they found the girl's breathing to be very shallow. She died at Dixie Regional Medical Center.

The mother was also suffering from carbon monoxide poisoning and was treated at the hospital.

Retracing the tragedy, McInness said the mother and daughter were travelling from Las Vegas to Provo, where the woman has a job.

"En route from Las Vegas to Provo, she hears a loud noise in the engine," he said. "She pulls off into St. George where she has a son."

The woman stopped at a couple of car repair shops, which confirmed that her exhaust system had broken. One did not do exhaust repairs, the other suggested that the system might be under warranty or under recall. The woman told the mechanic she would call a car dealership the next day.

The mother and daughter later drove to an apartment complex where the woman's son lived, but he was not home.

"They're sitting in the vehicle because the son is not here," McInness said. "It's the middle of the night, it's dark and it's cold. She fires up the car for a few minutes to warm them up."

Police were not releasing the name of the victims on Thursday, pending notification of family members. The two are originally from Tucson, Ariz. McInness said the investigation is still under way, but they believe the mother did nothing criminally wrong or negligent.

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E-mail: bwinslow@desnews.com