What are the illegal guns being obtained by Mexican drug gangs?

Of course the media are picking up on Eric Holder's concerns about Mexican drug gangs getting their guns from the US. This is one example of a piece from the WSJ.

This week, an Arizona gun shop goes on trial in state court in what law-enforcement officials are calling a landmark case against gun dealers who sell weapons that end up in the hands of Mexican drug cartels, fueling horrific violence south of the border that killed more than 6,000 people last year. . . . .

I have already noted that if you can't stop the gangs from smuggling drugs, how are you going to stop them from getting the guns to protect their valuable drugs? But here is a piece that claims that the types of guns used by the gangs are not the types of guns sold to civilians in the US.