The Police also said the 1997 BMW that she was found in may have a feature that prevents the doors from being opened from the inside once the car is locked with a key.
Investigators said they also found the horn did not work when the car was not on.
Police said 14-year-old Graciela Martinez had been taken to the hospital by her brother after he discovered her dead inside a vehicle Wednesday afternoon. Investigators said the girl had gone with her brother to school at Madera South High School. He had an early class and she stayed behind in the vehicle because her class started an hour later.
Police said after school, the brother went back to his car to find his sister in the back seat.
Stay with Action News and ABC30.com for more on this story.