In the period 1939 to 1941 , during the first years of World War II , the monastery at Stolobny Island , about 10 km ( 6.2 mi ) north of Ostashkov , was a prisoner-of-war camp of the Russian secret service NKVD , which held approximately 7,000 Poles who had been taken captive by the Soviet Union as a result of the Molotov – Ribbentrop Pact . Between April 3 and April 19 , 1940 , 6,311 Polish officers from the POW camp were executed by shooting as part of the Katyn massacre , and buried near Mednoye
Katyn massacre
Katyn massacre
Ostashkov