Abstract: A mold for use in fabricating a gas turbine fan casing out of composite material includes a mandrel for having wound thereon a fiber preform of a fan casing and an annular wall with an outside surface of profile corresponding to the profile of the inside surface of the casing that is to be fabricated and of two lateral rims of profiles corresponding to the profiles of outer flanges of the casing that is to be fabricated; a plurality of mold cover angular sectors that are assembled in leaktight manner on the mandrel and that are to close the mold and to compact the fiber preform wound on the mandrel; and a plurality of locking keys configured to lock together in leaktight manner neighboring mold cover sectors. A method of closing such a mold, and a method of fabricating a fan casing are included.