I was never 100% positive of anything in this case, other than that little Caylee had died under suspicious circumstances. And that, yes, Casey was the most likely suspect to my mind. Still there was so much strangeness surrounding this case, to include the fact that GA, an ex-LE, did not bother to contact the police when he even claimed he smelled decomp in the car when he went to pick it up while at the same time telling the tow company attendant his daughter was missing, and the rather odd exchange between Casey & Cindy regarding the pool ladder, to the plethora of lies that seemed part and parcel of that familiy's dynamic, that I could not be 100% sure Casey was solely responsible for the death of her child, much less guilty murder in the first.
That said, I am reminded of something John Douglas mentioned in an interview regarding the JBR case. And that was, those investigating that case approached it as if they were investigating a drug case as opposed to a homicide case. The primary difference between these two types of investigations is that in the case of drugs, detectives identify a suspect and proceed to build a case, whereas homicide detectives identify multiple suspects and proceed to rule them out.
I, personally, think that is what occurred in this case. They identified Casey as the sole suspect and proceeded to build a case to support their theory, as opposed to ruling out *all* possible suspects. and I think this is what hurt them in the end.