Think Sum of All Fears. In the movie, the nuclear material in the blast was quickly identified as Russian, to auto-generate a response. But the true signature of the material was shown to have originated with the US.
U1 sells some material to Russia that is on the books, and some to Syria or Iran that is off the books but from the same source. Later, when the weapon is smuggled into the US and is detonated, it will be shown to be the same as the material we sold the Russians.
So Russia again becomes the scapegoat and the object of joint retaliation by the EU. The question is, Putin must have known he was being played….so why did he go along?