Think of just 10 suicide teams of 2 members each, with nothing but hand grenades and ak-47's. Not that very hard to procure. At 3PM EST, they all enter the food courts of the 10 most crowded malls in america, where they procede to toss 3 hand grenades each, and mop up the rest with the ak 47's. It would be a freaking blood bath. It just takes the will, and operational secrecy. In fact there is just about nothing to prevent two lone nut muslims from doing this at a busy location, other than fear of losing their life.
That is the only thing that keeps the wolf at bay. The technology, feasability in a country with as lax security (relatively speaking) as ours is easy enough for an amateur to pull it off, as long as they are willing to say hello to the virgins.