According to State Department cables, the Mexican drug cartels are getting their weaponry from an international operation, with the cartels getting military weapons through various government channels. Some of those weapons did, in fact, come from the United States, but they weren't the result of the "gun show loophole" straw-man purchases or any of the other individual criminal acts anti-gun groups would have you believe.
Large quantities of those US weapons -everything from rifles to machine guns, grenade launchers, explosives and ammunition, came from purchases by the government of Mexico from the United States. As the underpaid, undermanned and undermined soldiers of the Mexican army skipped out on the military to put their training to work for the cartels, they took their issued-arms with them.
I guess the antigunners will have to come up with a new lie to tell.