I don't think you have to choose between one source of danger or the other. Sure, some terrorists are already in place, living and working in various parts of our country. Others have come in from Canada, with Seattle and Detroit as probably the major entry points. Others are coming in from Mexico.
This is partly a Border Patrol problem, partly a Homeland Security problem, but we shouldn't let our guard down with either. Yet we surely are.
We also need to watch out for prison conversions and for radical mosques and madrassahs that seek to make youthful converts to jihad.
Total agreement with you. I just think people who say Al Queada are coming to America because of "open borders" are not seeing it like you (or I) do. Their goal is to kill Americans, not to test the boundaries.
Probably one of those "secret" jails is where they're being held. Hope so, anyway.
The Border Patrol is part of the Department of Homeland Security, in the same sub department as Customs (They've integrated their air components already, called CBP Air). Immigrations and Customs Enforcement (ICE) and Transportation Security (TSA) are also part of DHS.