Given a Riemannian 3-ball (B, g) of non-negative scalar curvature, Bartnik conjectured that (B, g) admits an asymptotically flat (AF) extension (without horizons) of the least possible ADM mass, and that such a mass-minimizer is an AF solution to the static vacuum Einstein equations, uniquely determined by natural geometric conditions on the boundary data of (B, g).We prove the validity of the second statement, i.e. such mass-minimizers, if they exist, are indeed AF solutions of the static vacuum equations. On the other hand, we prove that the first statement is not true in general; there is a rather large class of bodies (B, g) for which a minimal mass extension does not exist.