Placing blame isn't really all that important. Making sure none of this happens again for the same reasons is.
If I was placing a bet, I'd say Wuhan researchers regularly got a handle on patents zero for cross species infection. In the course of the research a virus infected workers because of lax, sloppy, or otherwise inadequate controls; then despite the threat in order to save face government did everything they could to hide the mistake until it was far too late for anything to really be done about it.
The lancet letter was at best extraordinarily premature.