and a lot of it is done with the knowledge and permission of the BATFE.
that's a hell of an over statement. ATF walked guns make up a pathetically small amount of firearms in mexico.
For the uninitiated
@Filthy is referencing the ATF allowing guns to transit mexico in attempted sting operations. Some of these have gone all the way to cartels who then killed mexican police and others with them. The US has provided mexican army guns and they have deserted bringing their weapons along with them to their new cartel positions. This is all a small drop of the total guns in mexico. Not "a lot".
Mexican guns are largely American straw purchases, some stolen guns, and increasingly cheap chinese crap.
The 70%+ of mexicos illegal guns are from the US. Tracing those back to the straw purchase is almost impossible due to lack of universal background checking or any other type of database helping law enforcement.
Buy a gun, send it to Mexico. Without a sting operation selling that gun and targeting the suspect it is nearly impossible to prove.
Police can find that gun in Mexico and there's no tracking back to anything but manufacturer and gun dealer.
"Oh I sold that gun at a gun show" is all you have to say to cover a straw purchase.