"Border Patrol checkpoints do not give Border Patrol Agents carte blanche to automatically search persons and their vehicles, other then in the manner described above. In order to conduct a legal search under the Fourth Amendment, the agents must develop particularly probable cause to conduct a lawful search. Probable cause can be developed from agent observations, records checks, non-intrusive canine sniffs and other established means. Motorist's may consent to a search, but are not required to do so."
https://help.cbp.gov/app/answers/detail/a_id/1084/~/legal-authority-for-the-border-patrol
"Probable cause" is always the keyword!
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