Sonnenstein had been a fortress before it was anything else. In 1811 the site was converted into an institution for mentally ill patients considered curable, and under its first director, the physician Ernst Gottlob Pienitz, it earned a reputation across Germany for reform-minded psychiatry — the idea that people in distress could be treated rather than warehoused. The complex grew through the nineteenth century, and by 1928 it held more than seven hundred patients. That was the year Hermann Paul Nitsche was appointed director, and the institution's purpose began to invert. An advocate of what the regime called "racial hygiene," Nitsche pushed chronically ill patients out of the system, carried out forced sterilisations, and rationed food to people with conditions he classified as hereditary. In the autumn of 1939 Sonnenstein was closed to the public by order of the Saxon interior minister.

The programme that took over the site was run from a villa at Tiergartenstraße 4 in Berlin, and the address became its codename: T4. Sonnenstein was one of six killing centres established across Germany in 1940 and 1941, in which roughly seventy thousand people were gassed — patients taken from psychiatric hospitals, nursing homes, care homes and ordinary wards. In a screened-off section of the grounds, a gas chamber was built in the cellar of Haus C 16 with a crematorium attached. A wall already ran along the Elbe side; on the remaining sides they put up a high board fence, because Pirna was directly below and people could see. The doctor in charge was Horst Schumann. His successors worked under invented names — Dr. Storm, Dr. Bader, Dr. Palm, Dr. Friede — which tells you plainly that everyone involved knew what they were doing.

Buses came several times a week from institutions across Saxony and beyond. Between the end of June 1940 and the beginning of September 1941, 13,720 people were murdered in the cellar at Sonnenstein. They were people with schizophrenia and epilepsy and learning disabilities, people who had been in care for decades and people admitted the previous month, and every one of them had a family somewhere waiting for a letter. Among them was Elfriede Lohse-Wächtler, a Dresden painter whose work now hangs in German museums, and Martin Gauger, a church lawyer brought from Buchenwald. In 2010 a writer named Daniela Martin published a short book about her own great-grandmother, Anna L., born 1893, killed 1940 — one woman restored to a family, out of thousands who are still numbers. Hitler ordered the first phase halted on 24 August 1941. A further 1,031 concentration camp prisoners from Buchenwald, Sachsenhausen and Auschwitz were killed at Sonnenstein afterwards, under the programme designated 14f13.

This is the part that makes Sonnenstein more than a local atrocity. The killing centre was dissolved in the summer of 1942, and about a third of the people who had worked there — drivers, orderlies, stokers, clerks, nurses — were sent during 1942 and 1943 to Bełżec, Sobibór and Treblinka. They did not arrive as recruits. They arrived as experienced personnel who had run a gas chamber and a crematorium on an industrial schedule and knew how to move large numbers of people through a building without panic. The method used to murder disabled Germans on a hill above Pirna became the method used to murder Jews in occupied Poland, transferred by the same hands. Whatever the regime told itself it was doing at Sonnenstein, its actual output was a trained workforce.

The site stayed busy for half a century and silent about why it mattered. An Adolf Hitler School and a Wehrmacht hospital occupied it until 1945, then a refugee camp and a police school. From 1954 a factory built aircraft turbines on the grounds, and a rehabilitation centre opened in 1977 — which in 1991 became a workshop for disabled people run by the charity Arbeiterwohlfahrt, on the ground where disabled people had been killed. The reckoning began on 1 September 1989, when townspeople mounted a small exhibition on T4 by the historian Götz Aly. Public response produced a citizens' initiative, then a society, then archaeology: the cellar rooms were excavated between 1992 and 1994, reconstructed in 1995, and opened as a permanent exhibition on 9 June 2000. Outside, an installation by the Berlin artist Heike Ponwitz called "Vergangenheit ist Gegenwart" — the past is the present — sets each panel against Bernardo Bellotto's painted view of the old fortress, labelled with the regime's own vocabulary: collective transport, letter of condolence, special treatment, bathroom. The brief had been a memorial for roughly fifteen thousand people.