I have a possible other hypothesis outside of a dirty bomb...
Thr dirty bomb is growing some legs. But I have a possible other deal that might be interesting.
Dirty Bombs are more bark than bite. They are more of a scare tactic. Most Dirty Bombs in reality will not cause major damage. Not enough organization.
With that said, the scope and size of the search seem to be increasing. Leads me to believe, it's much more than 3 wayward medical devices with not enough radio-activity to harm much. Seems like some officials sent the medical device story to appease some, but might be a lot more than they are letting out. It might not be radioactive at all, it may be biological. Who knows.