You don't need a large number of terrorists to create the kind of devastation we saw on 9-11, all you need is opportunity.
And again -- to reiterate the same state-the-obvious point yet again until it sinks in..... you don't need to immigrate to do that either. It would be insane to sit through all that process and all that time and trouble when all you needed was to be in a plane, with someone on your team trained to navigate it, in American airspace. NONE of that requires citizenship. All it takes is a passport.
This is where Argument from Emotion shoots itself in the back eighty-seven times, pausing only once to reload. Once it's thought about rationally there's no there there.