Flat Lorentz 3-Manifolds and Cocompact Fuchsian Groups
  This paper gives a new proof of a result of Geoff Mess that the linear holonomy group of a complete flat Lorentz 3-manifold cannot be cocompact in SO(2,1). The proof uses a signed marked Lorentzian length-spectrum invariant developed by G.Margulis, reinterpreted in terms of deformations of hyperbolic surfaces. 
William M. Goldman and Gregory A. Margulis