"From December 2006 through July 2010, the Mexican government reported almost 30,000 deaths in Mexico resulting from organized crime and drug trafficking, with 9,635 murders in 2009 alone. In its fiscal year (FY) 2010 to FY 2016 strategic plan, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) reported that Mexico’s drug traffickers have turned aggressively to the United States as a source of guns and routinely smuggle guns from the United States into Mexico. This is, in part, because Mexican law severely restricts gun ownership.
"In 2009, ATF reported to Congress that about 90 percent of the guns recovered in Mexico that ATF has traced were initially sold in the United States.7 The Southwest border states – Texas, California, Arizona, and to a lesser extent, New Mexico – are primary sources of guns used by Mexican drug cartels. The growing crime rate in Mexico, and fears that the violence will spill over into the United States, have led to efforts by U.S. and Mexican authorities to attempt to curb firearms trafficking."

Source

"Review of ATF’s Project Gunrunner" (Washington, DC: US Dept. of Justice, Office of the Inspector General, Evaluation and Inspections Division, November 2010), p. 1.
http://www.justice.gov/oig/re…