  We perform an unbiased search of the origin of the recently observed 28
events above ~30 TeV in the IceCube neutrino observatory, assuming that these
are (apart from the atmospheric background) of astrophysical origin produced by
photohadronic interactions. Instead of relying on the normalization of the
neutrino flux, we demonstrate that spectral shape and flavor composition can be
used to constrain or identify the source class. In order to quantify our
observations, we use a model where the target photons are produced by the
synchrotron emission of co-accelerated electrons, and we include magnetic field
effects on the secondary muons, pions, and kaons. We find that the lack of
observed events with energies much larger than PeV points towards sources with
strong magnetic fields, which do not exhibit a direct correlation between
highest cosmic ray and neutrino energies. While the simplest AGN models with
efficient proton acceleration plausibly describe the current data at about the
3sigma confidence level, we show that IceCube can rule out that the observed
neutrinos stem from the sources of the ultra-high energy cosmic rays with a
factor of ten increased statistics at more than 5sigma if the current
observations are confirmed. A possible caveat are sources with strong magnetic
fields and high Lorentz factors, such as magnetic energy dominated GRBs.
