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.
And as usual they're fighting the last war. The (western) world has moved on from the fairly indiscriminate bombings of the 70s-90s. Targeted attacks either against specific persons (e.g. 2017 baseball shooting) or against members of groups (e.g. Summer 2020 street violence) are the New Hotness(TM).
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.