A briefcase filled with documents relating to a new Franco-British drone project has been stolen at the Gare du Nord station in Paris, reports say.
Police have been examining closed circuit film to discover who took the papers, reportedly on 2 February.
UK PM David Cameron discussed a plan for pilotless drones with President Nicolas Sarkozy last week.
French developer Dassault Aviation says no sensitive documents were taken and the project’s security is not at risk.
The BBC’s Christian Fraser in Paris says the bilateral development plan is the biggest since the two countries recently agreed to share defence assets.