If it isn't in an offline wallet it's likely with a legal entity registered somewhere or at least doing business in the US giving US courts jurisdiction. Even i

If it isn't in an offline wallet it's likely with a legal entity registered somewhere or at least doing business in the US giving US courts jurisdiction. Even if not in the US, whatever jurisdiction they are under can enforce US rulings or court orders if allied or under treaty. If the FBI/etc follow the process to get a judge's signature saying the entity is holding illegally obtained or fraudulent assets of any kind, the entity can be liable if they do not cooperate. Same thing as any order to seize or freeze financial assets but perhaps with more scrutiny given the pseudo-anonymity of digital currency. Same for acts by Congress that route through the Treasury to freeze assets like the Magnitsky Act.