years ago attended an EMS conference where one of the speakers was one of the top gang experts in the US, to my surprise, all gangs stem from two groups "The People", or "The Family". More surprising than that was that all gangs are Satanic in nature, hence the types of symbolism.
Unfortunately that conference was in the early 90's, and not much info on that speaker out there. It was at a time when there was a bunch of gang activity popping up locally and across the county, it was kind of interesting history lesson on gangs, and what types of graffiti and what not to keep eyes out for, since there were a bunch of "wannabe gangs" popping up too, could determine what was real and what may not be the real deal violent types