Depending on what type of radioactive material. Hospitals/Universities/Dentists offices have all sorts or radioactive material that are not Uranium. Cobalt would be the most likely material taken from a hospital/dentist office to make a dirty bomb imo. Which idk if we can track where the cobalt came from.If American universities and hospitals than trace elements are easily identified. ?
But it would be near impossible to put a blame on someone as that sorta material isn't as secure and abundant across everywhere in the states.