"Illicit trafficking across the Southwest border continues to be a chronic threat to our Nation and one of the top homeland security priorities for the United States. Transnational criminal organizations in Mexico dominate the illegal drug supply chain, taking ownership of drug shipments after they depart South America and overseeing their transportation to market and distribution throughout the United States. It is estimated that approximately 90 percent of the cocaine that is destined for U.S. markets transits the Mexico/Central America corridor. Mexico is the primary foreign source of marijuana and methamphetamine destined for U.S. markets and is also a source and transit country for heroin. Transnational criminal organizations based in Mexico dominate the U.S. drug trade--not just in border areas, but throughout much of the Nation. These organizations also control the south-bound flow of drug-related bulk currency and illegal weapons."
Office of National Drug Control Policy: http://www.whitehousedrugpolicy.gov/