[Azred]

A domestic terrorist doesn't have to worry about how to sneak into the country or how to blend in--they are already here and are already blending in. True, most people from such groups talk rather than act, but given half a chance they would strike at viable targets.
Besides, someone turning against their own country and/or people is, in its own way, worse than a non-native wreaking havoc. This is why treason (which is what you could classify domestic terrorism) is one of only two crimes mentioned in the Constitution.

This topic begins to show just how unsecure this nation is--like many nations are--despite the best attempts of the government and its Dept. of Homeland Security. Even if they were given 10 times their current resources they could cover only many things before stretching themsleves thin. Look at the current border problem with Mexico--the Minutemen have made a difference, but even with their help the Border Patrol cannot stem the tide of people crossing the border.
Honestly, I don't think the government can make even one city totally secure, much less the entire nation. I also think they know this, but refuse to say so because many people would subsequently panic. Simply collecting everyone's lighters before boarding a plane will make no difference.