A recent convert to Islam who told people he wanted to commit jihad and explode hand grenades in a mall was arrested in Chicago trying to buy hand grenades. The FBI learned of the plan from an alarmed acquaintance of the guy (who certainly seems a lone wacko to me) who then cased the mall with him, looking for places to plant them, while reporting back to the FBI. So, it appears this was a genuine threat.
They arrested the guy through solid police work, not bizarre governmental monitoring schemes scanning everyone’s email and phone records or insisting that carry-on plastic bottles at airports be put only put in zippable baggies, etc. No, they got him the old-fashioned way; a tip, followed by months of work – which seems the best, most effective way, as well as being the least harmful to civil liberties.
The mall, and the arrest, were not in Chicago. This incident took place in Rockford, IL, about 80 miles west.