"poor Latin" immigrants have many disadvantages, which is why they depend on coyotes. Presumably, a terrorist organization could:
* pay well enough that the coyotes would forget their principles
* provision their agents well enough to cross on their own.
Or, leave Mexico aside. Why not Canada? Why not Cuba? via boat from Haiti, landing in some god-forsaken part of Louisiana? I can't accept the proposition that getting into the country presents any serious difficulty for someone with means and motive.