  A re-examination of the atmospheric cascade profile of the highest energy
cosmic ray is presented. The study includes air-shower simulations considering
different cross sections, particle multiplicity and variation of the
hadronic-event-generator to model interactions above 200 GeV. The analysis
provides evidence that a medium mass nucleus primary reproduces the shower
profile quite well. This result does not support the idea, increasingly popular
at present, that the highest energy particles are protons, derived from the
decay of supermassive relic particles. On the other hand, we show that debris
of relativistic super-heavy nuclei, which can survive a 100 Mpc journey through
the primeval radiation are likely to generate such a kind of cascade.
