Not true, the French Customs authority (DGDDI) have pretty much immediately after the crash tweeted that none of their personnel were involved, and Le Monde is reporting the flight was conducted for the DGSE, which is the French external intelligence agency (similar to CIA or MI6). The Le Monde article also points out that the Maltese governement "thought" the flights being conducted over the last 5 months were customs flights, but that in fact the French Defence Minister had to admit that this was not the case...
Le crash de Malte lève un coin du voile sur les vols de la DGSE