...just for some additional background material on the subject, here are some decent gang assessments published by USAID last year:
Rising crime is threatening democratic development and slowing economic growth across Central America and Mexico. Gang activity has transcended the borders of Central America, Mexico, and the United States and evolved into a transnational concern that demands a coordinated, multi-national response to effectively combat increasingly sophisticated criminal gang networks.

Recognizing that gang activity is a complex, multi-faceted and transnational phenomenon, the USAID Bureau for Latin America and the Caribbean Office of Regional Sustainable Development (LAC/RSD) initiated the Central America and Mexico Gang Assessment in 2005 to study the phenomenon and propose solutions in five countries—El Salvador, Guatemala, Honduras, Mexico, and Nicaragua.
Central America and Mexico Gang Assessment - Full Report April 2006