I think the parents were the target and this was carried out by someone that loved and cared for Jayme. My theory would involve something that was said which spiralled out of control and led to this. I believe Jayme is a victim in this and in this scenario still alive and that is what we all pray for.
The lack of timeline for James, if he worked that day, what time he returned, was him working the reason for not attending the party. I think this is all really important for this case, not to mention the camera set up after. They must have felt someone may come back, why and who? Surely someone local.
I believe LE know or at least have a very good idea.