I mean local police. Not all customers lives in States.
The FBI will certainly be interested and they do have an awful lot of US customers. It was US regulators who fined them even though they're (likely to be) nowhere near the US. My local police are better equipped for chasing escaped sheep than online criminals.
Bitfinex is based in Hong Kong, IIRC. Remember hearing a lot about how bitcoin is "unregulated" there when the attack happened. But yes, the FBI always likes to get involved in big scale cyber crime cases, a figure I heard was if more than $100,000 dollars is involved then they want to hear about it. And at least that amount was stolen here so we'll probably see them somewhere.