I think Sid's point is that, following a military accident (which this clearly isn't), all documents for the aircraft would be immediately impounded so that any evidence would be preserved as it was at the time of the crash.
More difficult to ensure with a private operator and more difficult therefore for the AAIB to guarantee no interference post crash with the documentation.
I am NOT saying this could be the case with this sad accident but I think there is scope in the aviation community to 'lose' dodgy paperwork post accident.