While we're sharing theories and playing with speculations, allow me to speculate a bit:
Prior to his election, Donald Trump wanted to build a hotel in Saudi Arabia. The biggest builders in Saudi Arabia are, or at least were, Osama bin Laden's family. In doing business with the bin Ladens, (that is, assuming he did any business with them), Donald Trump might very well have had to give them money or received money from them. Given the size of the projects Trump does, this would likely be a considerable sum of money — say enough to finance a few Al Queda activities around the world?
Donald Trump refused to show us his tax returns.
That's all I have to say. I of course draw no inferences.
The New York Crank