We have a good idea of what happened.
Why and how are the interesting questions.
The FDR and CVR may not answer these questions but there's also the quick access recorder and the phones/tablets of crew and passengers to work with.
Phones/tablets timestamp almost everything they do and this can be extracted forensically. If the passengers asphyxiated there will be no user-initiated activity from that point on. If the passengers knew there was a problem there will be photos/videos showing this.