They found the body in the back seat of the car parked near Roza Dam and the Yakima Canyon around nine.
Investigators think the victim is from Yakima.
They got a call from another driver who spotted the car off the roadway.
"And I lifted him up. I was able to move him a little bit, so I knew he hadn't been gone too long. But he had been gone long enough that I couldn't bring him back," said Kathy Tucker who saw the body.
Kittitas County's Major Crimes Unit is now handling the case.
Detectives haven't said if the person is a man or a woman. They are withholding the name until they notify the family.