The well-known Nilov Monastery is on Stolobny Island , about north from Ostashkov . It was the place where the '' Ostashkov Special Camp '' of the NKVD was and where roughly 6,300 Polish policemen and prisoners of war were kept before they were executed in Tver . Approximately 4,300 of their comrades , held in Kozelsk , were around this time executed in Smolensk Oblast , in what is now known as the Katyn massacre
Katyn massacre
Katyn massacre
Ostashkov