Stan This is a direct copy, in part, of the DHS statement regarding their 100 mile border search and seizure policy.
The border search exception is a doctrine of United Statescriminal law that allows searches and seizures at international borders and their functional equivalent without a 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).
Stan, You asked for more details on the 100 mile border areas set up by the DHS. I don't really have much more than I put in my blog; however I do have the capability to research it a little further and will get back to you.