Being a former law enforcement officer from Atlanta, (and being one of the gang specialist for the department) I am familiar with gang problems... This even occurred in the smaller cities around Atlanta. As far as the US Marshalls Office... I don't think gang enforcement is a normal thing they deal with. It wasn't a thing they dealt with in the ATL anyways...
The sheriff will never admit to a gang problem. Just like many of the police chiefs in the smaller Atlanta suburbs wouldn't admit to having a gang problem in their towns. Some said it was boys being boys or "wannabees" or whatever else they wanted to lay it off on...
That is, until it was too late and it was time to play "catch up". It takes admitting there is a possible problem, knowing the one's who are involved from the start of the problem and monitoring the situation as it goes along and handling things as they progress. You will not be able to totally abolish the gang issues, but you can keep them controllable.
I have seen the tags on walls and signs and different things around... Those who keep saying there is not a problem, well, keep telling yourself that if it makes you feel better...
I know better... It may not be a major "problem" right now, but it's the making, trust me.
Just my opinion, I'll get off the soap box now...