ABC News "Nightline" went into the world of gangs in El Salvador, exploring the violence that keeps communities there living in constant fear and the impact of these deadly gangs as they spread north into the United States. Watch the full story on "Nightline" on May 17. Shown here, a prison outside San Salvador that holds inmates only from the 18th Street Gang; 169 members of its Revolucionario faction are packed into three cells.
Adam Desiderio/ABC News