When I was in Kharkov, Ukraine, my host drove me up to the Russian border where we visited these mass graves. It was very eerie. This is the site of the Katyn Massacre, where the Red Army executed over 20,000 Polish prisoners, many of whom were senior officers in the military that were captured in 1939.
Those who died at Katyn included an admiral, two generals, 24 colonels, 79 lieutenant colonels, 258 majors, 654 captains, 17 naval captains, 3,420 NCOs, seven chaplains, three landowners, a prince, 43 officials, 85 privates, and 131 refugees. Also among the dead were 20 university professors (including Stefan Kaczmarz); 300 physicians; several hundred lawyers, engineers, and teachers; and more than 100 writers and journalists as well as about 200 pilots. In all, the NKVD executed almost half the Polish officer corps. Altogether, during the massacre the NKVD murdered 14 Polish generals. I got this information (and you can get more) at from the Wikipedia page on the Katyn Massacre.