In the non-foreign-touristed-area we visit in Central Mexico, where they don't have problems with gangs yet, there are now groups of professional thieves. They can slit a purse/backpack, grab what's inside and be gone so swiftly that the victim isn't aware until it's too late to yell. It happens usually when the victim is distracted somehow or in a group perhaps being jostled a bit. I feel sure the technique will find its way to the most unsuspected places but at least people aren't being hurt in the process. With knives being used as tools of the trade, it's probably only a matter of time until someone bleeds. The thieves watch people coming out of banks and ATMs or they have a "lookout" who reports to the actors where the cash is.