I'm not going to list all the pieces here. No doubt someone else already has.
I think the flawed premise behind your question is the idea that there had to be some scientific forensic evidence that precisely identified Casey as the killer. There was none that did that, but there was plenty of evidence that Casey murdered the child.
The jury's job was to consider all the evidence and decide whether there was any reasonable doubt (not irrational doubt) that anything else could have happened. Casey killing this child was the ONLY reasonable explantion, period. That's why there was no reasonable doubt.
You don't need DNA or a fingerprint in the right place to get there. That's just wanting the forensic lab to do the job for you. Yes, it's great when we can get that kind of evidence. But people were getting convicted for murder before anyone know what DNA and fingerprints were.
Thank you, people seem to forget that.