The FBI was pursuing Hamzi and Mihdhar in August 2001 as possible suspects in the USS Cole bombing. All the hijackers had valid visas, Mihdhar being the last to arrive in July, so I'm not sure where you get "in the country illegally". INS had had the opportunity to stop each and every one as they entered but had (apparently) no reason to do so.

I should have written convicted criminals, obviously they were guilty of conspiracy but I think one has to be charged to legally qualify as a potential criminal. AFAIK it has not been definitively established if one or more were using alibis but no criminal records were found when issuing visas. I hope. (NB. Binalshibh the likely number 20 had been denied a visa.)

The acid test is: if a similar 'clean' group of terrorists, armed with valid visas, presented themselves at a border today what would happen ? Yes, they would be fingerprinted and photographed, then what ? Detained - why ? Turned away - on what basis ? Put on the NFL - maybe ?

They came in under the radar. The radar may be a bit better now but it's far from ironclad.