From what I hear every nuclear centrifuge creates a unique signature in its radiation and is easily traceable as to how it was made leading to where it came from.

What I am trying to say is Russia and the United States are the only countries able to produce High Quality plutonium ( say P-34 ). If this device was made of plutonium of a lesser quality we would know by the type of radiation in the cloud and what country is capable of producing it.