1. Either the motive was
sexual, or
2. She was
run over by a car (bad visibility and abrupt turns at the stretch between "bridge" and the intersection before the Academy) and the driver panicked
- Perhaps underage
- Perhaps drunk
- Perhaps a repeat offender
and decided to hide the accident.
If run over and her injury was serious but not fatal, the perp may have thought of driving away, leaving her there, but
- She might have been moaning loudly and generally would draw attention very quickly -> someone with a car could chase after him, or report having just seen his car drive by,
- She would be taken to hospital, possibly recover, and would be able to describe the perp and his car to police.
The so-called "stab" wounds may in fact be due to the car accident (hard to tell, her body is found severely decomposed) and the mouth and nose asphyxiation may be due to the killer wanting to silence her/finish her off.