Probably the Secret Service. They investigate wire fraud among other things.
If it's a security, then FBI should investigate.
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These crypto coins feel more like a security to me, so my bet is FBI.
In either case, fraud is rampant and there needs to be more crackdowns. So I don't care who investigates really. There just need to be more of them.
0. https://en.wikipedia.org/wiki/United_States_Secret_Service