Where every Criminal syndicate from the Bloods to the Crips to the Latin Kings actually USED prisons as a means to gather recruits and train them. Most people would be shocked to realize that the American prison system is in effect, a recruiting and training area for criminals, part University and part Cult compound.
Take some small time crooks, maybe some kids busted for weed. Throw them in with an ORGANIZED GROUP of criminals, whose groups literally have international chains of command, written constitutions, and infrastructures both in and outside the Prison walls. Once in, make the petty crooks have to do anything to survive in an overcrowded place where supervision is impossible, which of course makes the guards either corrupted, or frustrated, which means hyper violent. Make it IMPOSSIBLE to survive unless you join a gang, and difficult to survive even if you do join a gang. Then throw in the facts these gangs have means of training and indoctrination that would make most cults jealous, because they can take advantage of the fact people are isolated from family and in constant, 24/7 fear.
Make sure they are literally slaves of experienced criminals, who will make sure they need to know everything they need to know when they hit the outside, from simple criminal skills to talents like the ability to kill your best friends because the Boss wants you to. The major gangs do this like an apprentice program, think of a perverted version of Union apprenticeships. Add in nice touches like prison rape, which is really all about breaking the spirit of someone (same reason the CIA just got caught doing it in Iraq.) In the end, you have a taxpayer funded, often privately owned, means to turn out experienced criminals, most of which costs a lot more than to send someone to college.
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