  Downflows at supersonic speeds have been observed in the transition region
(TR) above sunspots for more than three decades. These downflows are often seen
in different TR spectral lines above sunspots. We have performed a statistical
investigation of these downflows using a large sample which was missing
earlier. The Interface Region Imaging Spectrograph (IRIS) has provided a wealth
of observational data of sunspots at high spatial and spectral resolution in
the past few years. We have identified sixty datasets obtained with IRIS raster
scans. Using an automated code, we identified the locations of strong downflows
within these sunspots. We found that around eighty percent of our sample show
supersonic downflows in the Si IV 1403 {\AA} line. These downflows mostly
appear in the penumbral regions, though some of them are found in the umbrae.
We also found that almost half of these downflows show signatures in
chromospheric lines. Furthermore, a detailed spectral analysis was performed by
selecting a small spectral window containing the O IV 1400/1401 {\AA} and Si IV
1403 {\AA} lines. Six Gaussian functions were simultaneously fitted to these
three spectral lines and their satellite lines associated with the supersonic
downflows. We calculated the intensity, Doppler velocity and line width for
these lines. Using the O IV 1400/1401 {\AA} line ratio, we find that the
downflow components are around one order of magnitude less dense than the
regular components. Results from our statistical analysis suggest that these
downflows may originate from the corona and that they are independent of the
background TR plasma.
