There is no meaningful way for any other country to hold any global superpower "accountable" unless they consent to such treatement. Nuclear weapons are a thing.

China has closed the book on this discussion. There's no further steps to take on the matter, even if you had unassailable proof, unless you wish to go to war.

Nobody wants to go to war (like that, anyway - the CIA may well wish to step up the trade war, based on how much anti-China sentiment is being stoked in the US population over the last dozen years).