Did anyone ever see the movie 'Manitou?'

Manitou is a pretty good movie that stared Tony Curtis and a few other people where an ancient Shaman begins to grow on this old flame of Curtis' and he seeks help from another Shaman, beautifully played by Michael Ansara, and he explains the concept of the Manitou, which is the life force or spirit in every thing natural and man made. Worth seeing.

I am not saying this is the answer, but if the mound dwellers existed on that land or where killed, it could be that their Manitous were absorbed (for lack of a better word) into the creatures that are also mound builders such as ants, crickets, etc. And, once inside those insects or creatures the Manitou of the Native Americans who lived there never died, but were reborn over and over again. The movie Manitou explains this better than I can. There is some part where the woman and a client of Curtis' repeat the same phrase that means, "My death foretells my life" meaning that the Shamans spirit could take physical form by using a person just like it used the woman in the movie to grow from her, on her back. It was a good movie for it's day.

I don't know if this helps, but it is a belief by Native Americans and might help explain the spiritual aspect or supernatural aspect of the movie. The magic of the Native Americans was very powerful and I wouldn't want to get on the bad side of any.

Hope this can give some insight into some ideas of how these creatures were able to "possess" this girl. If not, then I am sorry. Just something to keep in mind. And please, don't kill the messenger, I am only explaining what I know regarding the concept.