My first visit to the Aktion T4 killing site at Brandenburg an der Havel was in autumn. My destination, where 9,000 disabled people were murdered as part of the Nazi “euthanasia” program, is embedded in the activities of the town — trams and buses, stores, a bank, a cafe.

The buildings that were once the old prison were mostly destroyed during the war. If not for dark gray letters painted on one side of the light gray building — GEDENKSTÄTTE, on one side, and its English translation, MEMORIAL, on another — the site could easily be passed unnoticed. From a distance, it looks prefab, temporary, perhaps an ad hoc extension to an overcrowded school or municipal department.

Though it was October, I was thinking of winter. At the Nuremberg “Doctors’ Trial” in 1947, Viktor Brack — the economist, SS officer and head of the office of the Chancellery of the Führer who was in charge of Aktion T4 — testified that the first of the mass murders of disabled people happened “in snow-covered Brandenburg on a winter’s day in December 1939 or January 1940.” The exact date of this “test killing” has not yet been determined.

No documents from the “test killing” have been preserved. According to information at the memorial, “Who the murdered patients were and where they came from is unknown.” What is known comes primarily from postwar testimony of those involved, or thought to be involved, in what took place that day.

Unlike the Holocaust, there are no T4 survivors. We know about T4 and its aftermath mainly through medical records and from the perpetrators. Aktion T4 does not have its Elie Wiesel or Primo Levi.