It is a playbook, no matter who is doing it now. A dozen terrorists in 6 major cities doing this exact same thing. Multiply the current situation by even just this much, and increase the terror to all of us by 100 fold. Will not be surprised to see it happen.
I also pondered this possibility over the weekend. I agree that this could be a playbook. However, if the terrorists (if that is who they are) in the DC area are some of their better-trained ones, they could do a lot of damage in the DC area, creating a high note of terror, and then simply carry out successful (single) incidences in several other cities (with lesser-trained terrorists) at the same time to create sufficient terror effect without multiple killings in those cities.