The border search exception is a doctrine of United States criminal law that allows searches and seizures at international borders and their functional equivalent without a warrant or probable cause.
This doctrine is not actually an exception to the Fourth Amendment, but rather to the Amendment's requirement for a warrant (or probable cause). Balanced against the sovereign's interests at the border are the Fourth Amendment rights of entrants. Not only is the expectation of privacy less at the border than in the interior the Fourth Amendment balance between the interests of the Government and the privacy right of the individual is also struck much more favorably to the Government at the border. This balance at international borders means that routine searches are "reasonable" there, and therefore do not violate the Fourth Amendment's proscription against "unreasonable searches and seizures".
Border searches more invasive than "routine" require some suspicion. See the section below.
Despite federal law allowing certain federal agents to conduct suspicionless search and seizures within 100 miles of the border, the Supreme Court has clearly and repeatedly confirmed that the border search exception applies only at international borders and their functional equivalent (such as international airports).
The United States Customs and Border Protection (CBP), ICE-HSI Special Agents, and Coast Guard officers (E4 grade and above) who are all customs officers (those tasked with enforcing Title 19 of the United States Code) with the United States Department of Homeland Security, are permitted to search travelers and their belongings at the American border without probable cause or a warrant. Pursuant to this authority, customs officers may generally stop and search the property of any traveler entering or exiting the United States at random, or even based largely on ethnic profiles.
Although routine searches of the persons and effects of entrants are not subject to any requirement of reasonable suspicion, probable cause, or warrant, more invasive searches or seizures of a person's body require some suspicion.
The Supreme Court has held "that the detention of a traveller at the border, beyond the scope of a routine customs search and inspection, is justified at its inception if customs agents, considering all the facts surrounding the traveller and her trip, reasonably suspect that the traveller is smuggling contraband in her alimentary canal." (emphasis added) Characterized in terms of the Fourth Amendment, the Court was saying that such a detention ("seizure") was "reasonable", and therefore did not violate the Fourth Amendment. (The federal agents in this particular case did not X-ray ("search") her because she claimed she was pregnant. They instead decided to detain her long enough for ordinary bowel movements to evacuate the alimentary canal, despite her "heroic" efforts otherwise.)
The Supreme Court expressly did not rule what level of suspicion would be necessary for a strip, body-cavity, or involuntary x-ray search, though they did say that the only two standards for Fourth Amendment purposes short of a warrant were "reasonable suspicion" and "probable cause" (rejecting a "clear indication" standard).
In the border search context, reasonable suspicion means that the facts known to the customs officer at the time of the search, combined with the officer's reasonable inferences from those facts, provides the officer with a particularized and objective basis for suspecting that the search will reveal contraband. To form a basis for reasonable suspicion, a customs officer may rely on his training and prior experience, and may rely on entirely innocent factors, if the totality of the circumstances provide the officer with reasonable suspicion.