I'm currently reading The Good War by Studs Terkel. Terkel interviews a guy named Red Prendergast who was captured during the Battle of the Bulge.

Prendergast tells of arriving at the prison and being told that the captives were going to have a shower. Prendergast was worried that they were going to be gassed, because he had heard rumors of this happening to Jews and political prisoners in extermination camps. Per the interview, Prendergast was captured on December 19, 1944.

I was under the impression that most allied soldiers were unaware of the existence of concentration camps prior to coming across them in early 1945.

How early, and how widespread was knowledge of concentration camps among allied soldiers in Europe in WWII?