Am I the only one who heard "his mother?" It was said she might have left early or come home early and left. So why not consider her a suspect?
You don't just assume that a mother and father did it if there is any chance someone else with a key could come and go. This was sloppy and they simply did not want to do the job. Another case is a lot like this, if anyone remembers it. A young girl forced into modeling by her mother, murdered over Christmas.