"19 bad guys came in - legally - ... " This needs qualification, as you correctly point out. Legally, in this instance, refer to form rather than substance. They broke many laws, told many lies, and failed to follow rules. But each of them had a passport and visa that were apparently valid when they entered the Country as commercial passengers.
Frightening, isn't it?
"But each of them had a passport and visa that were apparently valid when they entered the Country as commercial passengers."
NO, I dont believe that is even the case. Some had overstayed visas is my understanding.
You'd probably can get the skinny on that from Michelle Malkin. here's a sample column with the various violations and abuses of the 245(i) and other loopholes by terrorists:
http://www.townhall.com/columnists/michellemalkin/printmm20020802.shtml I'd have answered sooner or more exactly on this point, but got caught up reading the amazon.com review of Malkin's "Invasion" book:
http://www.amazon.com/exec/obidos/ASIN/0895261464/%20vdare/103-8802220-9269437 wow.
Moreover, you can point to the John Lee Malvo case as one where the INS "catch and release" led to deaths later.