As German forces advanced in 1941, the Soviet NKVD executed thousands of political prisoners held in western Ukraine, Belarus, and the Baltic states rather than allow them to be liberated. Estimates range from 10,000 to more than 40,000 killed in a matter of weeks.
The massacres were documented by German forces for propaganda purposes and later by historians. Soviet authorities never acknowledged the killings. Post-Soviet archives and local memorials have partially recovered the record.