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.