I've always thought the parents did it. In UK law, when anything like this happens they always investigate the parents, and in a large percentage of cases it's the parents to blame.
The mother participated in beauty contests/pageants when she was younger, but was never very successful. She made her daughter re-live her dreams by having her take part in pageants. Mother spent a lot of time with daughter away from home going to pageants and not a lot of time with husband.
My theory is either a) Husband was resentful of mother spending too much time with daughter and killed daughter, or b) Mother was jealous of daughter's success that she never attained herself and so killed daughter.
I read a bit about the case several years ago and it seemed so obvious that one of the parents were to blame, but it seems US law is different from UK law and they never bothered investigating the parents properly.