Not only did 'these German citizens' know - they had friends and relatives in other citites... they ALL knew. Freaky little monsters...
At least one of the US generals forced the local gentry to tour the nearby camp after liberation. The magnitude of the crimes was laid out in front of them, and resulted in suicides here and there. In the book “Interrogations”, one of the SS flunkies who operated the gas chamber in one of the camps said, to his interrogator (who was a turned German) how bad the operator booth smelled, but how after a while, one could eat a sandwich in there.
A Jewish child in Poland who was taken by his parents to stay with a Gentile family, friends of theirs, was at a grocery with his “adopted” family, and listened to the grocer laugh about how the child’s entire village, including his parents, had been rounded up and hauled away. He never saw anyone from his childhood again, including any member of his family, and it took him some years before he had any trust or love for anyone.