I would say that that began sometime in the 1970s, as television programmes started to take a greater interest in what had happened and Holocaust deniers began to emerge to suggest that no such atrocities had taken place.We should remember that the collapse of Nazi Germany was latterly an organised collapse, but was rapid in its early stages. However, it was not so rapid that the Nazis were not able to dismantle and destroy evidence of camps, of which Treblinka was one, as Stewart Maxwell mentioned.