The "we already know who did it" is, at this stage, more speculative. Naming parties without full understanding of what's going on can cause it's own damage, an

The "we already know who did it" is, at this stage, more speculative. Naming parties without full understanding of what's going on can cause it's own damage, and make a diplomatic solution harder to obtain. The evidence gathered to date (including what they think it might be) has the potential to disclose, or at least insinuate, what capabilities the U.S. has to determine what's going on. Infuriating as it is, handling it carefully is probably the best course of action here. Often that means having to be coy with what's known until there's a certain degree of certainty.