Mass graves were very common - have a great-great aunt that is likely buried in one at a state mental hospital in an adjoining state. Only problem is that ‘medical privacy’ is so restrictive I’d have to get a court order just to see the records. The burial ground is posted with no trespassing signs, which is itself illegal in that very same state.
Good Lord alone knows the sketchy stuff that happened in these places.
There was an article in the Chicago Tribune some years ago about things that went on at that place on the northwest side. Human experimentation, torture, general neglect. Some bloodcurdling stuff.