I haven't seen it discussed (which kinda disturbs me), so here goes.
Please google "operation gun runner". There are too many developments to try and explain it in a post. It is still unravelling and being exposed. In a nutshell, the ATF purposly sold weapons in border states and stood aside while they were carted to Mexico. This was supposedly done so tracking them to drug lords would be easy. It has since been exposed as a plot to attack the 2nd Amendment. The ATF seems to be crumbling from the top down (might be a good thing).
It also appears that Obama was in on it from the start.
The truth, the whole truth and nothing but the truth!