  Using the most advanced formulation of the hadron resonance gas model we
analyze the two sets of irregularities found at chemical freeze-out of central
nuclear-nuclear collisions at the center of mass energies 3.8-4.9 GeV and
7.6-9.2 GeV. In addition to previously reported irregularities at the collision
energies 4.9 GeV and 9.2 GeV we found sharp peaks of baryonic charge density.
Also we analyze the collision energy dependence of the modified Wroblewski
factor and the strangeness suppression factor. Based on the thermostatic
properties of the mixed phase of a 1-st order phase transition and the ones of
the Hagedorn mass spectrum we explain, respectively, the reason of observed
chemical equilibration of strangeness at the collision energy 4.9 GeV and above
8.7 GeV. It is argued that the both sets of irregularities possibly evidence
for two phase transitions, namely, the 1-st order transition at lower energy
range and the 2-nd order transition at higher one. In combination with a recent
analysis of the light nuclei number fluctuations we conclude that the center of
mass collision energy range 8.8-9.2 GeV may be in the nearest vicinity of the
QCD tricritical endpoint. The properties of the phase existing between two
phase transitions are revealed and discussed.
