  Magnetic reconnection is believed to be the dominant energy release mechanism
in solar flares. The standard flare model predicts both downward and upward
outflow plasmas with speeds close to the coronal Alfv\'{e}n speed. Yet,
spectroscopic observations of such outflows, especially the downflows, are
extremely rare. With observations of the newly launched Interface Region
Imaging Spectrograph (IRIS), we report the detection of greatly redshifted
($\sim$125 km s$^{-1}$ along line of sight) Fe {\sc{xxi}} 1354.08\AA{} emission
line with a $\sim$100 km s$^{-1}$ nonthermal width at the reconnection site of
a flare. The redshifted Fe {\sc{xxi}} feature coincides spatially with the
loop-top X-Ray source observed by the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI). We interpret this large redshift as the
signature of downward-moving reconnection outflow/hot retracting loops. Imaging
observations from both IRIS and the Atmospheric Imaging Assembly (AIA) onboard
the Solar Dynamics Observatory (SDO) also reveal the eruption and reconnection
processes. Fast downward-propagating blobs along these loops are also found
from cool emission lines (e.g., Si {\sc{iv}}, O {\sc{iv}}, C {\sc{ii}}, Mg
{\sc{ii}}) and images of AIA and IRIS. Furthermore, the entire Fe {\sc{xxi}}
line is blueshifted by $\sim$260 km s$^{-1}$ at the loop footpoints, where the
cool lines mentioned above all exhibit obvious redshift, a result that is
consistent with the scenario of chromospheric evaporation induced by
downward-propagating nonthermal electrons from the reconnection site.
