Graciela Martinez was found unresponsive by her two siblings inside the 1997 BMW at the end of the school day on Wednesday. They rushed her to the hospital but it was too late. Early autopsy results indicate the girl likely died from "environmental hyperthermia," or heat stroke, reports ABC News Fresno affiliate KFSN-TV.
The girl's brother, Oscar Martinez, says the three drove to school together that morning, and that Graciela stayed in the car because her first class didn't start for another hour.
"She must have fallen asleep, and then right after school we found her. My sister found her, she was all dried, white," said Oscar Martinez, crying.
So far, there does not appear to be any sign of a struggle.
After inspecting the car, police say the vehicle may have a feature that prevents the doors from opening from the inside once the car is locked. Investigators also say Graciela may have been unable to honk the horn without a key in the ignition.
KFSN-TV contributed to this report.