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The extragalactic sources of ultra-high-energy (E > 4x10^19 eV) cosmic raysthat make a small contribution to the flux of particles recorded byground-based arrays are discussed. We show that cosmic rays from such sourcescan produce a noticeable diffuse gamma-ray flux in intergalactic space comparedto the the data obtained with Fermi LAT (onboard the Fermi space observatory).A possible type of active galactic nuclei (AGNs) in which cosmic-ray protonscan be accelerated to energies 1021 eV is considered as an illustration of suchsources. We conclude that ultra-high-energy cosmic rays from the AGNs beingdiscussed can contribute significantly to the extragalactic diffuse gamma-rayemission. In addition, a constraint on the fraction of the AGNs underconsideration relative to the BL Lac objects and radio galaxies has beenobtained from a comparison with the Fermi LAT data.

Contribution of Cosmic Rays from Sources with a Monoenergetic Proton Spectrum to the Extragalactic Diffuse Gamma-Ray Emission