It would appear the immediate priority is to ensure permanent preservation of ALL records, documents, evidence, transcripts....etc.....guaranteed by each and every authority in possession of same.
Then, another and final investigation take place that undertakes to fully document everything related to this tragedy....create a Time Line and Chronological review of Statements, Documents, etc to document the exact chain of events that transpired.
It should address each issue, disputed point, decision, and action taken by the various authorities to determine what actually transpired before, at the time of the accident, and afterwards.
Let the Chips lay where they fall....and if there is blame to be fixed.....do so without partiality and favor.
Just set forth what is known and how it all relates to everything else.
But....the very first step has to be the preservation of the documentary evidence.