We show that a group acting properly discontinuously on a complete simply connected riemannian manifold of pinched negative curvature and fixing an ideal point must be finitely generated. Thus any discrete virtually nilpotent subgroup of isometries of such a manifold is necessarily finitely generated. In the course of the proof, we introduce a notion of rotational part for parabolic isometries.
J. Differential Geom. 38 (1993) 559-583.