apparently the flight was part of a French customs surveillance operation.
Sources in France are speaking about 3 French military and 2 civilians ,( the pilots of the leased a/c ) Ministry of defense confirmed the death of 3 of its personal.
Another source ( Ouest France ) indicate the a/c was leased from Luxembourg by the French Civil intelligence service DGSE and Military Intelligence DRM . Looking at the intended destination this does not sound like Customs surveillance.