Border Patrol would be only one agency that might pinch someone for smuggling contraband across the border. At the border crossing, there are Customs special agents and inspectors. Formerly, these agents and inspectors were with the US Customs Service (USCS), which is now under the new Department of Homeland Security along with INS. The newly created "BICE" is an acronym for the Bureau of Immigration and Customs and Enforcement.
Since the September 11, 2001 attacks, BICE went into a hiring mode and substantially increased the number of inspectors to work at the previously porous border crossings and ports of entry. The primary function of these federally trained, armed inspectors is to detect contraband entering the US and to arrest the offenders. Inspectors like ripping things apart: packages, containers, crates and especially your shiny, new SUV. And because you're crossing the border, they need no probable cause, not even reasonable suspicion to search. In fact, they can even search you for hemorrhoids if they so desire. (Alright, just kidding about the hemorrhoids, but I still cringe when I see Customs and TSA screeners begin to cermoniously slap on those rubber gloves.)
In all seriousness, even if one were to get past the border crossing, the BPA maintains interior border check points. And as if all that were not enough, there are still the state and local police.
Rick Collins does an excellent job of covering the legal consequences of a border crossing, among other things, in his book, LEGAL MUSCLE. There is the case study of Dianne Wright, a grandmother with a medically determined condition, who declared her medications, which were controlled substances, to Customs pursuant to the personal use exemption. The package was stamped by Customs and she was cleared at the border crossing to go on her way. She was later stopped in Texas by a local sheriff for a traffic violation. What followed next is truly disturbing.
RW