Findings detail that 70% of the guns recovered from crime scenes in Mexico from 2009 to 2014 and traced by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), originated in the United States, Motherjones reported, detailing how cartel gun runners often buy weapons legally in the United States, either at gun shops, gun shows, or in private sales and smuggle them across the border.
Additionally, cartels buy unassembled parts for firearms and ship them to be built across the border. US retailers do not have to report when they sell gun parts or kits. To get around that, the smugglers will purchase “80% receivers” which only require a small amount of additional work to be completed after purchase. Manufacturers in the US are not required to stamp serial numbers on gun parts, leaving completed weapons untraceable.
"They are also easy to conceal," the US Government Accountability Office report notes of the gun parts, "making it more challenging for customs authorities to detect illicit shipments of such parts."
The ATF, tasked with apprehending smugglers, has long had problems with understaffing and underfunding. Last week, Obama addressed the problem with executive actions that include adding 200 new ATF agents and investigators.