Albert Speer, Hitler's Armaments Minister, was one of the twenty-two defendants at the trial of the major Nazi war criminals in 1945-1946. He was charged on all four counts of the indictment (planning aggressive war, crimes against peace, war crimes and crimes against humanity). The Tribunal acquitted him on counts 1 and 2, and found him guilty on counts 3 and 4. He was sentenced to twenty years imprisonment.
New evidence would appear to indicate that he was indeed present at the Posen speech of October 6, 1943 and must therefore have known about the extermination program. And the evidence comes from Speer himself.
The Holocaust History project has an article discussing this evidence in German and English. The article was written by Sven Felix Kellerhoff, and is translated into English by THHP's Gord McFee. The article may be found at the link below.