There is no doubt in the mind of any serious and reputable historian about who was responsible for the Katyn massacre. It was carried out by the People's Commissariat for Internal Affairs better known as the NKVD (Наро́дный комиссариа́т вну́тренних дел) at the direct order of the Politburo under Stalin. This has been established beyond any and all reasonable doubt by numerous independent investigations as early as 1943. The only reason the Western Allies entertained the Soviet narrative of the massacre being conducted by Germans is that at the time it was deemed more important to maintain good relations with the USSR as their contribution to the war effort was deemed more important. At no point did any of the allied leadership actually believe it was anyone but the NKVD who was responsible.

I wrote a bit about that not too long ago in this thread if you're interested.