  Sterile neutrinos with a mass around the keV scale are an attractive particle
physics candidate for Warm Dark Matter. Although many frameworks have been
presented in which these neutrinos can fulfill all phenomenological
constraints, there are hardly any models known that can explain such a peculiar
mass pattern, one sterile neutrino at the keV scale and the other two
considerably heavier, while at the same time being compatible with low-energy
neutrino data. In this paper, we present models based on the Froggatt-Nielsen
mechanism, which can give such an explanation. We explain how to assign
Froggatt-Nielsen charges in a successful way, and we give a detailed discussion
of all conditions to be fulfilled. It turns out that the typical arbitrariness
of the charge assignments is greatly reduced when trying to carefully account
for all constraints. We furthermore present analytical calculations of a few
simplified models, while quasi-perfect models are found numerically.
