Buchenwald concentration camp
One former Buchenwald prisoner, Armin Walter, counted the number of people who were killed by being shot in the back of the head.  Walter's job at Buchenwald was to set up and care for a radio system at the place where people were executed.  He counted the number of people who were killed and hid the information.  He says that 8,483 Soviet prisoners of war were shot and killed.