If there is a clear profile that is searched, all that bad guys have to do is find people who don't fit the profile to do the job.
See [link|http://swiss.csail.mit.edu/6095/student-papers/spring02-papers/caps.htm|http://swiss.csail.m...2-papers/caps.htm] for details.
In fact the terrorists did this in 9/11. A big reason why so many of the people participating were Saudi is that they knew that people from Saudi Arabia had expedited visa procedures. Secondly they actually did a dry run before the real one to verify that they wouldn't be caught.
Cheers,
Ben