There is a lot of hyperbole about this subject and some rather inflated claims about the sources of some of the weapons they say came from the US. Quite a few did come from the US,through the mexican government who did not hang onto them. Some went away when the soldiers defected to the cartels. Some went away when the cartels raided government armories and Police Stations. Some went across the border illegally as well but by far not the majority of them. In this case one of the guns that went across the border with the permission of ATF was involved in the shootout that resulted in the death of Border Patrol Agent Terry.