Nuclear reactors have unique signatures. Like fingerprints, each is individual and can be identified. If a nuclear device went off anywhere in the world, we could obtain samples and identify what reactor produced the weapons grade material. The problem is that in order to positively identify a particular reactor; it is necessary to have an identified reactor provide information to create the fingerprint in the first place. This could identify the origins of a nuclear device that a terrorist organization may deploy some day. This brings two potential problems to my mind:
1) What if the signature does not match any known reactor?
This is possible if say, Iran builds a nuclear device and does not allow our technicians to obtain a signature. It could be North Korean, or Syrian. It could even be some other country that we don’t know about.
2) What if the signature does match a known country? Let’s take Russia as an example. The device could have been stolen. Even if the government were part of the deal, they sure as hell would not admit it. How would we know?
Problems like this have been solid excuses for war. The destruction of the U.S.S. Maine was the trigger for the Spanish-American war. The cause of the explosion is still not known. However, the U.S. was looking for a fight, and it was an excellent excuse.
In the event of a nuclear weapon or weapons having destroyed one or more cities in the U.S., it is more than likely we will find ourselves in this type of situation. War against governments could be easily justified. It would be natural to choose governments that are covertly hostile to us already.