Way back in 2001, just after the second plane hit the second World Trade Center tower, before anyone in the US really thought much about terrorism, I turned to my girlfriend (who would later become my wife) and told her that it made sense that the second plane would wait a short while. It means that they get more media coverage. And it would not surprise me in the least if we would see something else a short while later (which we did). And so on.
As it turned out, there wasn't a whole lot more after the Pentagon plane hit, but there certainly could have been more. A whole lot more.