  We explore scenarios where the highest energy cosmic rays (HECR) are produced
by new particle physics near the grand unification scale. Using detailed
numerical simulations of extragalactic cosmic and gamma-ray propagation, we
show the existence of a significant parameter space for which such scenarios
are consistent with all observational constraints. An average fraction of about
10% gamma-rays in the total cosmic ray flux around 10 EeV [10**(19) eV] would
imply both a non-acceleration origin of HECR and a large scale extragalactic
magnetic field smaller than about 10**(-11) G. Proposed observatories for
ultra-high energy cosmic rays should be able to test for this signature.
