I'm not a lawyer, so I don't know if they COULD or not, but if they did could they PROVE it when they were not able to prove to a jury that she killed her? That would seem to be harder to prove than murder unless there was an eye witness to Casey being the one who disposed of the body or a confession from Casey that she did it. They had a lot of circumstantial evidence against her for murder, but none that I heard of for disposing of the body. No finger prints, no DNA...nothing to tie Casey to the disposal of the body. JMO
Nothing other than the small matter of a decomposing body in her car...