  We analyze the spectra of pions and protons in heavy-ion collisions at
relativistic energies from 2 A GeV to 65+65 A GeV by using a jet-implemented
hadron-string cascade model. In this energy region, hadron transverse mass
spectra first show softening until SPS energies, and re-hardening may emerge at
RHIC energies. Since hadronic matter is expected to show only softening at
higher energy densities, this re-hardening of spectra can be interpreted as a
good signature of the quark-gluon plasma formation.
