They weren't found in the car, they were found nearby. The car was submerged in a lake/pond.
So, the only scenario that fits the perp not doing it, is that they accidentally drove into the lake, and they died after succumbing to exposure even if they were able to exit the vehicle.
The simple fact is, the perp didn't have a Mustang to sell them, so he was, at the least, going to rob them. It's not much of a stretch to figure he killed them too, and unless there are marks on the bodies consistent with crashing into a lake, and not consistent with being murdered, the likelihood is he killed them.
I don’t think he did it, involved yes I think so, I just don’t think he did the deed.He came in with his dad and gave himself up, that’s not what murderers do.. I would almost bet others are involved.