See http://arxiv.org/pdf/hep-th/9605147v1.pdf. Most of the following is verbatim from there.
Theorem: With the following assumptions: G is a group of symmetries of the S-matrix S; G contains a subgroup P′0 that is locally isomorphic to P(r,1) for r≥3; all particle types correspond to positive-energy time-like representations of the universal covering group of P′0; the number of particle types is finite; at least locally, G is generated by generators represented in the one-particle space H(1) by (generalized) integral operators in momentum space, with distributions as kernels; the amplitudes for elastic scattering of two particles do not vanish identically; and the scattering amplitudes are regular functions of the momenta, and the amplitudes for scattering between two-particle states are analytic functions in some neighborhood of the physical region, we can state that G is isomorphic to the direct product of P′0 and the direct product of some internal symmetry group.
See the linked paper for the proof of the statement, if needed.