In 2010, a multi-step experimental program, the Beam Energy Scan (BES), was launched at the
Relativistic Heavy Ion Collider (RHIC) to investigate the phase diagram of strongly interacting
nuclear matter. The BES phase I (BES I) was completed in 2011 with Au+Au data sets in energy
range from 39 GeV to 7.7 GeV. This was complemented by earlier (62.4, 130 and 200 GeV) and
later (54.5 GeV) collected data sets of Au+Au collisions. Many measurements taken by the STAR
(Solenoidal Tracker At RHIC) detector at $\sqrt{s}_{NN}$ below the RHIC injection energy suffer from large
statistical errors, sharply increasing with decreasing energy. Nevertheless, they allowed for the
first time a direct study of the QCD critical point (CP) and phase transition signatures. The results
of these studies are presented. In 2015, the Beam Energy Scan program was extended to energies
below $\sqrt{s}_{NN}$ = 7.7 GeV by successful implementation of the fixed-target mode of data taking
(FXT) in the STAR experiment, in addition to the standard collider configuration. In the fixed
target mode, ions circulating in one ring of the collider interact with a stationary target at the
entrance of the STAR Time Projection Chamber. The first results from the exploratory FXT run
with Au+Au collisions at $\sqrt{s}_{NN}$ = 4.5 GeV are presented. The ongoing second phase of BES (BES
II), which includes both collider and FXT data taking, will provide data sets with one-order-of magnitude
larger statistics in collider mode, and with two-orders-of-magnitude larger statistics in
FXT mode at each of several energies down to $\sqrt{s}_{NN}$ = 3.0 GeV.