The powder is the evidence. The signature of the device can be analyzed and the origin can be determined in hours. At least, the point of construction could be determined.
If nuclear weapons go up for sale the way that MiGs and Blackhawk helicopters are, then where it was built matters a whole lot less than who set it off. That's the real danger of when nuclear arms become so common they're 'just another weapon'.
I am all for bombing them now and wiping out their capability to make war on anyone.