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We continue work initiated in a 1990 preprint of Mess giving a geometricparameterization of the moduli space of classical solutions to Einstein'sequations in 2+1 dimensions with cosmological constant 0 or -1 (the case +1 hasbeen worked out in the interim by the present author). In this paper we make afirst step toward the 3+1-dimensional case by determining exactly which closed3-manifolds M^3 arise as spacelike slices of flat spacetimes, and by findingall possible holonomy homomorphisms pi_1(M^3) to ISO(3,1).Comment: 10 page

3-manifolds which are spacelike slices of flat spacetimes