But it was pretty well known for the british and american governments(since 1941 or at the latest 1942) , and even to this day, some details of said knowledge(the intelligence reports on decryptions on concentration camps communication) are not availble to historians.
One has to wonder why.
Actually when reading more about it, even if the American public would have known, it probably wouldn't have mattered much, judging by later genocides :
""no U.S. president has ever suffered politically for his indifference to its occurrence[genocide]. It is thus no coincidence that genocide rages on."
http://www.amazon.com/exec/obidos/ASIN/0060541644/metafilter...
Also another book shows how the holocaust was an open secret , known to millions since at least 1941:
http://www.amazon.com/exec/obidos/ASIN/0805059849/metafilter...