“Unfortunately, we have had to give up Katyn. The Bolsheviks undoubtedly will soon ‘find’ that we shot 12,000 Polish officers. That episode is one that is going to cause us quite a little trouble in the future. The Soviets are undoubtedly going to make it their business to discover as many mass-graves as possible and then blame it on us” - Joseph Goebbels, private diary
I think one of the more confusing parts here, is that it was actually the British spreading these lies, FDR didn’t believe it and joked about it with Stalin, and Stalin used Churchill’s belief that it happened to argue privately with FDR that Churchill and the British were nazi sympathizers and as soon as the war was over they would turn traitor and become even worse than the fascists and nazis.
sorry couldn’t find a non paywalled source but this is the book and I’m pretty sure the right page
https://archive.org/details/goebbelsdiaries100goeb/page/316/mode/2up