In a bright room at the Wiener Library—the world's oldest Holocaust archive, in a building off Russell Square, London—the curator Barbara Warnock is telling me about the first generation of Holocaust researchers; men and women who risked their lives, during and after the war, to collect evidence of all kinds that bore witness to the horrors that they themselves often experienced.

It's the day before the opening of the library's exhibition, Crimes Uncovered: The First Generation of Holocaust Researchers. The walls are lined with pictures of people, places, books, and records. We look up at a middle-aged man in a suit, wide-shouldered, charismatic, a wry look on his face. A picture taken years after the war, years after horrors few of us can imagine.

Filip Müller was barely 20 years old when, in April of 1942, he was deported to the Auschwitz concentration camp. The young Jewish Slovak was a sonderkommando; part of a forced labor unit, he was ordered to burn the bodies of his fellow prisoners in the ovens. As far as the Nazis were concerned, Müller was good at the job, able to tell those who were about to be murdered that they were safe as he readied the gas chambers.

That—along with his extraordinary mental and physical toughness—played a part in ensuring his survival. But during his time in Auschwitz, Müller was doing something else: collecting evidence. Caught in a horrific, vulnerable position, the young man tried to kill himself when he realized that he had been facilitating the killing of thousands of Jews. He entered a gas chamber along with a group of fellow prisoners who were forced there, but, as he recorded in a memoir written years after the war, a little girl came up to him and convinced him not to do what he was doing.