  We discuss the GZK horizon of protons and present a method to constrain the
injection spectrum of ultrahigh energy cosmic rays (UHECRs) from supposedly
identified extragalactic sources. This method can be applied even when only one
or two events per source are observed and is based on the analysis of the
probability for a given source to populate different energy bins, depending on
the actual CR injection spectral index. In particular, we show that for a
typical source density of $4\times 10^{-5} Mpc^{-3}$, a data set of 100 events
above $6\times 10^{19}$ eV allows one in 97% of all cases to distinguish a
source spectrum $dN/dE\propto E^{-1.1}$ from one with $E^{-2.7}$ at 95%
confidence level.
