Actually, if Mexico can identify a potential responsible party, they would have pretty good grounds for extradition. Something the US Government could not just push aside.
Another possibility is something I was just reading about very recently. Apparently, there is an international law or treaty or something that lets foreign nationals sue US companies. The story was connected to Chiquita, the banana company, formerly US Fruit, involved in the banana wars. It seems in 2007 or so they had to settle with the US government for paying protection money to be left alone by FARC and various other Central and South American gangsters, drug cartels, and terrorists. The fedgov treated those payments as bribes or something and came after Chiquita saying they were "financing" these bad guys. Chiquita settled with the fedgov for lotsa millions of dollars.
Now, it seems certain American law firms are lining up people in these Central and South American countries injured by the gangsters, druggers, and terrorists to sue Chiquita under the international law or treaty.
I hope those gun stores that "cooperated" (ordered?) with Fast and Furious kept proof they were told by ATF to make the sale.