Might have been drugs? Mexico is the number one (by far) source of almost every kind of illegal drug into the US. More immigrants from Mexico are here working for the drug cartels to distribute their products throughout the US than are here to pick lettuce.
As you'll note on the monument map a Mexican highway paralells the park, within a hundred yards or so, blocked by a couple of strands of barbed wire (unless it's been changed in the last few years). Cross border drug drops are simple. It's another part of the US which Americans, at least law abiding ones, can't frequent.