Spoiler: It's parcel shipments.
Because narcos are smarter than people who blame immigrants for crap they don't do.
Why would they route through US which has a higher standard of screening rather than shipping it directly to Mexico?
What's is the import export certificate label on these boxes? US importers are registered with bonds.
Who picks up these boxes in the US?
What's with master carton? You can't really create cartons randomly. They all have to belong to same shipper.
What's the import code on these package? It's impossible to lie and smuggle stuff like these. You can say these are medical products then who is listed as American importer?
So expect more quality articles like this.
Did you also notice the subtle "Russian doll" anchor in the article?