A suitable response would be to present proof of what happened and who did it, then pursue law enforcement solutions and public shaming.

If this had been a land incursion, and the public Army had been the responding department, there'd be no question, and we'd have pictures of tanks and AK-47s on the front page of every newspaper. Congress would be discussing nothing else.

However, it's the spies, and FBI wannabe-spies that are probably handling this. The proof is not as obvious as a tank in North Dakota, but I suspect that there's is enough evidence and forensics to prove to the NSA and their ilk who did it and how.

But a spy's natural instinct is to hide the fact that they know something. It's a card up the sleeve, and their concern is more about being able to continue to spy than to stop a currently ongoing crime.

Well, they could confront the issue instead of publicly ignoring it for a change. Haven't heard much about China's massive DDoS against github. Even github won't point the finger at China even though it was clearly done at the Great Firewall.

The US is trying to establish norms for cyber operations in the international community - this is a missing and critical piece.

But the US does have cyberwarfare policies. As part of a deterrence strategy the US strikes back when it can attribute attacks to a specific party. You may also remember the recent Executive Order from the Obama Administration whereby a series of policies were joined so that the US is organized to levy sanctions against governments, actors and corporations overseas that are involved in sponsoring intrusions into US networks.

Do they have any centrifuges that need to be exploded? ;)