If it is a major terrorist group, I'm surprised it took this long. Seems fairly simple to rig something up and make it go poof. Wouldn't take much to shut down a major US city. We aren't equipped for it and we just aren't used to it.
A few car bombs here. A few attacks alla DC Sniper there. Five or six guys and a few thousand in financial backing could shut NYC down tight if they were halfway intelligent.