I'm getting into a very morbid and disturbing place that I would rather not go, but here goes....
Three points:
Hannah's skull was sent to richmond, indication that it was not attached to the rest of the skeleton. How easily does the skull typically become detached? Wouldn't there be some connective tissue holding it together or would have been eaten by vultures for sure?
Morgan's mother said she believed she died quickly. She knew the cause of death (per the link by dove28 above) There is a cause of death that is likely very (hopefully) quick, that could be gleaned from skeletal remains.
Cassandra, while he is not (yet?) linked to her death, an interesting fact is that her skull was detached.
http://www.newsplex.com/home/headlines/78135977.html?device=phone&c=y
Where I'm going here, is something I don't want to say or contemplate, but nonetheless could be the case.... Perhaps they were decapitated. Ugh, I can't even stand the thought, so heinous, but the fact that the skull was detached has led me to believe that could be the case. I see that as such an atrociously monstrous act...I can't even fathom how this man was in our midst for so long.