I wonder how they arrived here.
Probably legally like Tim McVeigh, or Muhammed Atta, the original WTC bombers, or that lily white CITIZEN Andrew Speaker with TB terrorizing the skies, or the ex-military DC Sniper, or the Cho sunhui kid at VaTech....or any number of totally LEGAL ways to get into this country.
If you're a petty criminal, you do stupid things like rob convenience stores--maximum risk, minimal payoff.
If you're a diabolical terrorist, capable of detonating suitcase nukes or destroying the WTC, you DO NOT risk being an undocumented flake. You merely get a legal tourist visa, fly into the country on American Airlines and check your suitcase nuke.
Let's spend all our money on a border fence and ignore the fact that super-badguys will always get in, and are already here legally anyway.