Abstract: Distinct preforms are made for different portions of a composite material part that is to be fabricated, and they are assembled together in a non-densified state or in a state that is not completely densified by uniting respective contacting surfaces of the preforms (13, 15, 36) together by means of fibrils (17, 19) projecting perpendicularly from the surface of one of the preforms (13, 15) and in which the surface of another preform (36) is engaged. The assembled-together preforms are then co-densified. The fibrils are formed on the surface of a fiber preform by the ends of fibers thereof that have been displaced by the preform being needled. The method is particularly suitable for fabricating a sandwich panel of composite material comprising two rigid skins between which there is disposed a core constituted by partitions that are perpendicular to the skins.