Police in California are investigating the mysterious death of a 14-year-old girl, who appears to have died after becoming trapped inside a car parked at her high school, unable to unlock the car's doors.

Graciela Martinez, 14, was found dead in her brother's BMW, parked in a lot at Madera South High School after school on Wednesday, police said. Graciela is believed to have fallen asleep in the car early that morning and then became trapped inside.

Found at the end of the school day, Graciela was slumped in the car's back seat and drenched in sweat. Police believe the girl died from heat exhaustion.

Investigators with the Madera Police Department discovered that when locked from the outside, doors cannot be opened from inside the used 1997 BMW.

"It appears there may be a defective locking system on the car but we still need to do further tests to determine if that's true and correct," Det. Robert Salas told ABC News affiliate KFSN-TV.

Furthermore, Salas said, it appears the car's horn did not work without a key in the ignition. Graciela had no food, water or a cellphone, according to police.

"The mother reported that she was healthy, never had any medical issues, never had any health issues. She was healthy the day she went to school," Salas said.