The entire article is pro-click but here’s the first section:
As we discussed in a previous analysis, the 90 percent number was derived from a June 2009 U.S. Government Accountability Office (GAO) report to Congress on U.S. efforts to combat arms trafficking to Mexico (see external link).
According to the GAO report, some 30,000 firearms were seized from criminals by Mexican authorities in 2008. Of these 30,000 firearms, information pertaining to 7,200 of them (24 percent) was submitted to the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) for tracing. Of these 7,200 guns, only about 4,000 could be traced by the ATF, and of these 4,000, some 3,480 (87 percent) were shown to have come from the United States.
This means that the 87 percent figure relates to the number of weapons submitted by the Mexican government to the ATF that could be successfully traced and not from the total number of weapons seized by Mexican authorities or even from the total number of weapons submitted to the ATF for tracing. In fact, the 3,480 guns positively traced to the United States equals less than 12 percent of the total arms seized in Mexico in 2008 and less than 48 percent of all those submitted by the Mexican government to the ATF for tracing. This means that almost 90 percent of the guns seized in Mexico in 2008 were not traced back to the United States.
The remaining 22,800 firearms seized by Mexican authorities in 2008 were not traced for a variety of reasons. In addition to factors such as bureaucratic barriers and negligence, many of the weapons seized by Mexican authorities either do not bear serial numbers or have had their serial numbers altered or obliterated. It is also important to understand that the Mexican authorities simply don’t bother to submit some classes of weapons to the ATF for tracing. Such weapons include firearms they identify as coming from their own military or police forces, or guns that they can trace back themselves as being sold through the Mexican Defense Department’s Arms and Ammunition Marketing Division (UCAM). Likewise, they do not ask ATF to trace military ordnance from third countries like the South Korean fragmentation grenades commonly used in cartel attacks.
Of course, some or even many of the 22,800 firearms the Mexicans did not submit to ATF for tracing may have originated in the United States. But according to the figures presented by the GAO, there is no evidence to support the assertion that 90 percent of the guns used by the Mexican cartels come from the United States — especially when not even 50 percent of those that were submitted for tracing were ultimately found to be of U.S. origin.