LetM be a simply connected, homogeneous space of nonpositive curvature, and letG be the isometry group ofM. In this paper we study the Lie algebra ofG; we describe the set of roots with respect to a particular abelian subalgebra, and its stability subalgebra at a special point ofM (∞). These descriptions are then used to give conditions which are equivalent to the fact thatM is a symmetric space of noncompact type; we also obtain a new criterion for the visibility property ofM, in terms of the action ofG onM (∞).