The government has a unique interest in policing its borders, and people have a lower reasonable expectation of privacy where borders are involved. So, in general, officials have more search-and-seizure leeway at and around the border.
Broad Definition of “Border”
Customs or immigration officials may conduct warrantless searches without probable cause not only at borders, but at their “functional equivalents,” such as international airports. Essentially, neither citizens nor noncitizens have Fourth Amendment rights in these situations.
For much more on border searches, see When can officials stop, question, and search travelers at the U.S. border?
Border officials may stop motorists at fixed checkpoints that are reasonably located relative to the border, to question motorists even in the absence of reasonable suspicion of a crime, let alone probable cause. (U.S. v. Martinez-Fuerte, U.S. Supreme Court, 1976.)
To learn a great deal more, see What are the search-and-seizure rules at permanent interior immigration checkpoints?
Officials on roving patrol generally need reasonable suspicion before they can stop a car.
For futher information on the roving patrol context, including the contours of “reasonable suspicion” and the power to search, see When can roving border patrols stop and question you?