  Secondary photons and neutrinos produced in the interactions of cosmic ray
protons emitted by distant Active Galactic Nuclei (AGN) with the photon
background along the line of sight can reveal a wealth of new information about
the intergalactic magnetic fields (IGMF), extragalactic background light (EBL),
and the acceleration mechanisms of cosmic rays. The secondary photons may have
already been observed by gamma-ray telescopes. We show that the secondary
neutrinos improve the prospects of discovering distant blazars by IceCube, and
we discuss the ramifications for the cosmic backgrounds, magnetic fields, and
AGN models.
