In Auschwitz, the prisoner resistance was led by Polish soldiers and patriots. They established close contacts with the Polish underground beyond the barbed wire. Through these channels the inmates received essential goods, such as medicine, and passed on evidence about SS crimes. In 1944, they even smuggled out photographs of the mass murder of Jews. The images had been taken by prisoners from the Birkenau Special Squad, to alert the world to the Holocaust.
We are sending you pictures from Birkenau, from a gassing operation. The picture shows one of the pyres in the open air, on which corpses are burned, when the crematorium cannot keep up with the burning. In front of the pyre lie corpses, waiting to be thrown onto the pyre. The other picture shows one of the places in the little woods, where people undress supposedly for a shower, and then go to the gas […]. The pictures can be enlarged and, we feel, be sent further on.
Source: H. Świebocki, The Resistance Movement, W. Długoborski and F. Piper (eds), Auschwitz: 1940–1945, vol. 4 (Oświęcim, 2000), p. 279