The 9-11 terrorists entered the US legally.
Technically, I guess. There was an article in National Review a couple of years ago on the passports they used. I can't find it on google (and I don't know that it was ever on the website; I read it in the print edition), about how at least a number of them never should have been allowed in on the basis of their applications. The only example I remember is that one put for his prospective USA address "California" (!). IIRC, the article made the point that those applying for entry on a Saudi passport got in pretty automatically. Wish I had hung on to that issue!
Not exactly true.
Some were on watch lists, and, as the Able Danger revelations show, not for random reasons.
Some committed visa fraud.
Some got visas only due to lackadaisy in our embassies.
Some had expired or otherwise invalid visas.
Some had encounters with police with invalid visas, invalid and/or fraudulent licenses, etc.
Some had encounters with the FAA for the love of God, doing things like leaving a rented plane on a runway, and STILL their immigration status was not checked.
Some made openly terroristic threats in the presence of federal government officials!
Had the immigration laws and regs IN PLACE AT THE TIME been enforced, 9/11 would not have happened. 9/11 is a direct product of lax immigration enforcement.