I think the intrel brouhaha brought about by Senator Robert Torricelli (NJ's greatest shame- oh, until our governor tried to court a man by giving him a domestic security job he was not qualified for) is seriously to blame for the mistakes such as this that allowed 9/11 to occur.
I agree that the CIA should not be investigating U.S. citizens, but "US persons"? Just because you enter the country legally does not mean you are above suspicion. If you are linked to a terrorist group, you should be investigated.
Hollow's anecdote is important- we screen our citizens for drug infractions more thoroughly than we screen our guests for terrorist ties.