Generalized Auslander conjecture for nil-affine crystallographic groups

Let $N$ be a connected and simply connected nilpotent Lie group, and let $\Gamma\subseteq\operatorname{Aff}(N)$ act properly discontinuously and cocompactly on $N$; such an action is called nil-affine crystallographic. Generalized Auslander conjecture. Then $\Gamma$ is virtually polycyclic. This asks for the analogue of the Auslander conjecture for nil-affine crystallographic groups. The source gives Dekimpe's converse result that every virtually polycyclic group admits a nil-affine crystallographic action, unique up to conjugation, but supplies no resolution of this converse implication.