Belle Mayer trained as a lawyer and worked for the General Counsel of the US Treasury, Foreign Funds Control Bureau. This bureau worked to enforce the Trading With the Enemy Act passed by Congress. In this capacity, Mayer became familiar with the German IG Farben chemical company, a large conglomerate that used slave labor during World War II. In 1945, Mayer was sent as a Department of Treasury representative to the postwar London Conference. She was present as representatives from the Allied nations outlined the principles of law for the prosecution and trial of Europe's major war criminals. Mayer reported to this commission as it prepared for upcoming war crimes trials. She was then among the attorneys (including her future husband William Zeck) who prepared the indictment against the IG Farben company at the Nuremberg trials.
It is estimated that in the construction of this enormous plant, in the construction alone, 25,000 workers died. These workers were very, very badly treated and the plant was run with the help of the SS. In other words, there were SS guards in every part of the factory. Not only that, but the SS brought the concentration camp inmates from Auschwitz to the factory every day and I.G. Farben gave them receipts. They paid them, they paid the SS a dollar for a man, a dollar a day, 75 cents for a woman, and 50 cents for a healthy child, the child that could work. Then the SS complained that it was too dangerous to move these people over such a long road, that there was a danger that they could escape, no matter how many guards they put on. So Farben, in conjunction with the SS, recommended that Birkenau be built. It was closer to Auschwitz proper and they built the crematorium, the gas chambers and the crematorium right into the building.