I think it's funny how some of these bureaus take themselves too seriously. ISIL probably doesn't care much about the new ATF headquarters, it's not a cultural landmark.
I'm not sure what tractable measures they could take (that's not my job to figure out and if it was I wouldn't do it for free) but I feel very confident saying that whoever or whatever group next takes a swing at government will likely do so outside of whatever measures protect the workplace.