That's a good question. A fair bet is that the perps would be found fairly soon. After all, nukes are special items, well researched. Each device would leave a distinctive signature pointing directly at it origin, and its path from factory to destination would be of interest to many investigative agencies.
I think 9/11 proved that investigating after the fact - like what happened after the first time jihadis tried to destroy the WTC, in 1993 - is not good enough. It must not be allowed to happen in the first place, not when it is every bit as predictable as the Iranians reneging on this deal with the EU.