I have a 20+ foot tall American Elm tree. I believe it was unharmed yesterday. Today the bark was stripped almost entirely around the several-inch-wide trunk and in various places up to a height of something like 15 feet. The top-most branches that were stripped were only a bit bigger than a pencil (and high in the air), but the trunk at the ground has large, serious looking claw marks.
I don't see the patterning on the wood I would expect from porcupines, and it's hard to believe a squirrel would have done that much damage (or left such big claw marks). I live in a suburb outside of Denver. I've included some pictures.
 Some additional information: It's a Frontier elm, so it's supposed to be resistant to Dutch elm disease. My neighbor says there's been a lot of raccoon activity lately, and my wife says she might have seen a raccoon or similarly sized gray animal near the tree that night. As far as I know, raccoons do not destroy trees like this though.