would you risk with fake documents planning such action?

Visas to enter US is given by U.S ambassady (US ambassador or consul) in any foregin country ( or not, mostly not ;-) )
anyway, if they travel to US before, they must have valid and legal documents to be there. If not , noone will let them pass gate at airfield.
No matter now what kind of vise that was (turist, student, bussines). But was confirmend by U.S gov ( or workers of goverment, such as imigration office )
Somone can be let in or not to US is decided by immigration office at airport wher eyou arrive. From country you fly to U.S mostly noone chk too deep you will get perrmission to cross border or not. Enough is you have valid visa given by ambassady or agreement between countrys let you enter plane travelling to US (like GB, Germany......)
When you arrive, guy on airfield decide , he let you in or not.
So if they not plan to land, noone can chk them to deep.
So, practical if they not land they cannot be illegals, cuz they not cross US border. (as i remember aircrafts are not exteritoriall)
but US gov will never tell you , what was true. And best explanation was they where illegals, cuz they get this documents using lie about they travel target (somone must be dumb to tell truth " we going to US to hit WTC " ;-)
thats all