Jimmy indeed considered a plausible possibility, but the scope was too vast. For those Americans who had experienced the Cold War, having some doomsday enthusiasts was quite normal. In fact, there were numerous companies that thrived on building doomsday shelters, and some even specialized in customizing survival tools and furniture needed for these shelters. In short, if this was their direction for the investigation, no matter how many people they had, it would be useless because many wouldn't disclose that they had a shelter.
Nevertheless, since they had already discovered the first shelter, more would certainly be encountered later, serving as an alternative direction, at least. It's unclear how Agent Raymond and his team would approach this.
