  Let ({\Sigma}, g) be a compact $C^2$ finslerian 3-manifold. If the geodesic
flow of g is completely integrable, and the singular set is a tamely-embedded
polyhedron, then ${\pi}_1({\Sigma})$ is almost polycyclic. On the other hand,
if {\Sigma} is a compact, irreducible 3-manifold and ${\pi}_1({\Sigma})$ is
infinite polycyclic while ${\pi}_2({\Sigma})$ is trivial, then {\Sigma} admits
an analytic riemannian metric whose geodesic flow is completely integrable and
singular set is a real-analytic variety. Additional results in higher
dimensions are proven.
