I'd always assumed it was Palmer (he's mechanically inclined and could know how to pick a lock).
I've also heard arguments that it was Windows. It goes like this: Windows is the one you see in silhouette about to be thing-ed by the dog. Later, you hear him drop something when he comes across Bennings being thing-ed, which are the keys he just used to break into the blood bank. (Even though Windows was a thing himself, he was still genuinely surprised that the burned remains were alive.) Palmer's attack didn't turn him into a thing, it just forced him out in the open. MacReady's test didn't work on Windows because he had thought of it long enough beforehand that he was able to prepare for it.
What do you all think?
I dont know....that was a p**ser. I think it was likey Windows.
I am lost, is this The Thing with the alien that comes from outerspace or something else?
Thanks,
John