Hi,
Is it the first time external personal are involved
Not at all ...
For the Concorde case the AAIB was involved ...
But the AAIB was under constraint and had no access to all evidences ..........
I have not a link .. but all this is explained in a report of the AAIB about their findings for Concorde crash.
If you can participate in a BEA investigation but can't access to some evidence .. what the heck ?
Hope this will not be the case for AF447