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 \(\mathscr{S}\); \(G\) contains a subgroup \(\mathcal{P}^\prime_0\) that is locally isomorphic to \(\mathcal{P}(r,1)\) for \(r\geq3\); all particle types correspond to positive-energy time-like representations of the universal covering group of \(\mathcal{P}^\prime_0\); the number of particle types is finite; at least locally, \(G\) is generated by generators represented in the one-particle space \(\mathcal{H}^{(1)}\) 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 \(\mathcal{P}^\prime_0\) and the direct product of some internal symmetry group**.

See the linked paper for the proof of the statement, if needed.