Didn't they make threats regarding "twin cities"?
Yeah, I believe so... I do not recall the exact information though.
Either way, Balloons are launched from every NWS office 2 to 4 times a day to take atmospheric readings for weather forecasting models.
It does make me think about the quote from Osama's Revenge though, “They also discovered illustrations running the length of the room that showed how high altitude balloons could be used to release large quantities of anthrax and bubonic plague bacilli over major metropolitan areas.”
In any event, I am pretty sure it is nothing to be concerned about.