I can totally see it being the Polish, but I don't think there is a clear finger that points to anyone (including the US and Russia.) This does sound more plausible than the US. If the US did it, it would literally be done with support of other NATO allies (including Germany itself, and in secret.) It's simply too risky for them to end up destroying the entire NATO relationship, which they would not risk at this juncture...so it doesn't really make sense. Poland on the other hand, would still be a buffer between the rest of Europe and Russia even if Ukraine falls...so they have a lot less to lose, because no one is going to really punish them. I still feel it as -probably- Russia, but I don't think there is enough evidence, and it's very likely we'll just never know. The US theory to me is just logically bunk.