<i>“You could surmise that [the parts] are coming from the US, since most of the weapons that come down here illegally are from the US,”</i><p>Perhaps, since the largest US source is M16s (and maybe M4s now) donated to the Mexican government and then stolen (second is Communist bloc stuff bought on the open market, it's much less expensive than making your own like this). There's not much utility in tracing any of these guns, except of course to propagandize for gun control in the US (see Fast and Furious for the most egregious example).<p>Which makes me wonder why this cartel felt a need to make their own AR-15s or M16s (the article didn't mention if these were 3 round burst or full auto, did it)? If they can get their hands on rocket propelled grenades....<p>Then again, this whole expose might be a lie (again, see Fast and Furious).