If they were interested in stopping another 9/11, there would be much more stringent screening of muslim immigrants and surveillance of all those muslim brotherhood owned and operated mosques here in the US that over and over again are linked to these terrorist attacks.
You have right on this issue too; thing is there are Muslim immigrants already in this country who need to be watched.
It would be so easy if the government would just do something like limit immigration from countries with a high incidence of Islamic radicalism.
If they wanted to avoid mentioning “Islam,” they could also use Freedom House rankings, and restrict immigrants from countries ranked as “not free” https://freedomhouse.org/report/freedom-world/freedom-world-2016/ and that encompasses every single Muslim country in the world.