Where do Mexican drug gangs get their guns?

Apparently, at least some of them come from U.S. narcotics control agents.  Not only should this qualify as a "jackbooted thug of the week" case for our friends at the Northern Muckraker, but it also serves as an example of why border security--and transitively control of the firearm and narcotics trade across the border--should not rely solely in the integrity of federal agents involved. 

Reality is that narcotics and BATFE agents can and are bought by drug and gun cartels.  However, a border fence, being inanimate, is a bit less susceptible to a bribe.  Even a vehicle barrier would do a lot to curb illegal trade in guns and drugs by making it more difficult to simply drive across the border in many areas, and would not hurt native wildlife.