IIRC, that is not the only known bombing on continental US soil during WW2, and the other events were Japanese attacks. It's been a long time, but I believe there were some helium balloon carried bombs that were launched and were known to hit a couple rural areas in Colorado and maybe Nebraska or Kansas. This topic was discussed in one of my WW2 classes at CSU, and I believe one of the bombs went off near Windsor, close to Ft. Collins and CSU. I believe the Japanese were experimenting with the jetstream and using balloon bombing. Kinda fuzzy on this...