A direct attack by a state actor (especially one like the US, which likes to be seen as following the rule of law) isn't too likely. China might attack bitcons, but why would they - they can just block them.

The people who do black ops don't generally do so to help the financial sector. They do so for national security. While the economy (and monetary system) is arguably a national security issue, I can't see them all being on the same page. The guys in treasury would probably rather talk to regulators than intelligence.

Simply banning them, and enforcing that ban would work though.