A Bieberbach theorem for crystallographic group extensions

In this paper we prove that for each dimension $n$ there are only finitely many isomorphism classes of pairs of groups $(\mathrm{\Gamma },\mathrm{N})$ such that $\mathrm{\Gamma }$ is an $n$-dimensional crystallographic group and $\mathrm{N}$ is a normal subgroup of $\mathrm{\Gamma }$ such that $\mathrm{\Gamma }\mathrm{/}\mathrm{N}$ is a crystallographic group.