Perhaps some of FT's Legal Beagles can weigh in because my memory is fuzzy. I recall reading that the courts permit the CBP to operate internal border checkpoints as "extentions" of the actual border checkpoints. In order to conduct a search, they must have reasonable suspicion or someone would need to be intimidated enough to consent. The courts came up with different definitions of "fixed" and "roving" checkpoints. There were (I think) four criteria for these checkpoints and for conducting a warrantless search:
1. They had to establish that you, your vehicle occupants, and your vehicle had actually crossed the US border. I always wondered about this when the I-5 and AZ internal checkpoints have multiple off amd on-ramps (like the entire city of San Diego!) long before you get to the I-5 checkpoint.
2. The trip, if it did include a border crossing, had to have been continuous. For example, crossing the border on Monday, staying overnight somewhere, and confronting a checkpoint on Tuesday doesn't count as "continuous."
3. I don't remember the other two criteria.
There is a huge temptation to use these internal checkpoints to catch people for all sorts of things for which they would normally need probable cause or an actual search warrant. If you have not crossed a border and the CBP did not establish that you had crossed a border, they can't conduct a warrantless search, but they do... This kind of dragnet scares the daylights out of me.