Earlier quoted context omitted.
What do you think would happen to a Mexican police officer and his family if he tried to simply "just do his job"? There is a reason the police are the ones wearing ski masks down there: http://si.wsj.net/public/resources/images/WO-AG531_MEXICO_G_...
How did it get to this level? We have criminal gangs in the US and the police here don't have to worry about that. It's mostly because of our justice system and the fact that you can't buy off any cop with some cash.
In other words have overlapping independent organizations all investigating each other and add in people that don't stay in an area long enough to be bribed safely.