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Unveiling the mystery of the sources of ultra high energetic cosmic rays (UHECR) that possess energies above 1018 eV is one of the most urgent questions in contemporary astroparticle physics. Up to now no final identification of the sources is achieved even after more than one century of research has passed since Victor Hess discovered cosmic rays in the year 1912. In this field of science which covers a vast energy range already first clues of sources can be made for low energies but at the highest energetic end the current status is not conclusive. In this thesis predictions for UHECR observables are made and a comparison with the data mainly from the Pierre Auger Observatory is carried out to constrain the properties of source candidates in this energy range. A number of different observables that measure a variety of properties of cosmic rays as the energy spectrum, the mass composition, the direction and the existence of UHE photons and neutrinos are used to ensure a holistic view on UHECR. The theoretical predictions of these observables from astrophysical scenarios including assumptions on sources and on the properties of the intergalactic space are employed to explain the observed data and consequently lead to new insights of the sources itself and the properties of the universe. The predictions are executed with the sophisticated Monte Carlo code CRPropa 2.0 that enable simulation of the UHECR propagation from putative sources to Earth by taking into account all relevant processes. With this procedure the discrimination of the mass composition, the energy spectrum and the maximum energy of the cosmic rays at the sources was possible. Additionally, also the distributions of the sources and the magnetic field are studied.