From NPR:
In September 1940, Pilecki didn't know exactly what was going on in Auschwitz, but he knew someone had to find out. He would spend two and a half years in the prison camp, smuggling out word of the methods of execution and interrogation. He would eventually escape and author the first intelligence report on the camp.
In the early years of the war, little was known about the area near the town Germans called Auschwitz.
Poland was in a state of chaos. It was divided in half — Nazi Germany claiming one side, Soviet Russia on the other. The Polish resistance had gone underground.
Pilecki wanted to infiltrate the Auschwitz camp, but he had difficulty getting commanders to sign off on the mission. At the time, it was thought of as POW camp.
“They didn't realize the information from inside the camp was that vital,” says Ryszard Bugajski, a Polish filmmaker who directed the 2006 film The Death of Captain Pilecki.
Pilecki was eventually cleared to insert himself into a street round-up of Poles in Warsaw on Sept. 19, 1940. Upon arrival, he learned Auschwitz was far from anything the Resistance had imagined.
More here.