Headless chickens massacred, but not dismembered or otherwise eaten? Small entrance opening? And you're in MN?
I'm betting a weasel, mink, otter or something along those lines. Second bet is a raccoon, although I'm guessing a distant second.
Either of these are some of the toughest preds to deal with. Clever and able to squeeze into and through some tiny spaces, they are both wily. Below ground wire mesh, cement footing edges, grade level planking and low electric wire are some of the better ways of coping with them.
Along with my personal favorites, traps and guns, of course...