The events in Katyn, Kozelsk , Ostashkov , Starobelsk, Jukhnovo , Yuzhe , Tyotkino , Kozelshchyna, Oranki, Vologda, Gryazovets are well documented. These were Soviet extermination centres for Poles who might resist the new partition of Poland. Katyn is simply the easiest as it's the best known, as far back as 1943. The German advance into Belarus uncovers the mass grave of Polish officers, priests, rabbis, lecturers etc. The Nazi's release this as a major propaganda coup, the Polish government in exile calls for an investigation by the International Red Cross. The Soviets cut off all communication with them, does anyone still wonder why the Soviet advance stalled outside Warsaw when the free poles launched an uprising ? The cover-up, yes obviously, the Soviets have destroyed the best of German armies, you don't upset a good ally. When the Cold War starts fingers start to point, only journalist I can find who called this out was Epstein. He basically said Voice of America had and was still covering up the facts because they're commies as well, was an age of McCarthyism remember.