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Nymphalis antiopa
It wouldn't open it's wings for me :( Spotted in University of Idaho Arboretum
I think you are correct. I don't believe there are any other butterflies in the USA with that color pattern except for Mourning Cloaks.
http://bugguide.net/node/view/3188
Without its wings being fully open, it's hard for me to be completely sure. But I think it is a Mourning Cloak.