The streets were almost deserted at midnight as a man pushed a trolley stacked with boxes through Mexico City. One tumbled off and cracked open slightly, giving a policeman who tried to help a glimpse of its contents - human remains. They were the dismembered bodies of two boys, aged 12 and 14. The case sparked horror in a country that is already troubled by massive violence and highlighted the role of children in the drug war. Alan and Hector, two boys of indigenous origin, were friends, living in Mexico City's dilapidated historic centre, a hunting ground for local cartels. They had last bee...