Somebody has to provide evidence (and, you know, a perpetrator) to a prosecutor first. But there's no real "investigation team", save maybe FINRA.

If FINRA detects something they can refer an investigation to the S.E.C. or other law enforcement body. But they don't exactly have a stellar track record. And nobody even has to listen to them; they just gather evidence and make recommendations. It's very much a dog with no teeth.