They came from across occupied Europe. Riwka Herszberg, aged seven, was from Zdunska Wola; Ruchla Zylberberg, eight, from Zawichost; Blumel Mekler, eleven, from Sandomierz; Marek James, six, and Marek Steinbaum, ten, from Radom. Walter Jungleib, twelve, was Slovak. Eduard Hornemann was twelve and his brother Alexander was eight, and they had lived at 29 Staringstraat in Eindhoven, where both their parents worked at the Philips factory. Their mother, Elisabeth, died of typhus at Auschwitz in October 1944; their father, Philip, died at Sachsenhausen in February 1945. Eleonora and Roman Witonski's father, Seweryn, had been a paediatrician in Radom before he was shot in the cemetery at Szydlowiec. These were children with addresses, siblings, birthdays and family trades - not a category.

Kurt Heissmeyer was an SS physician who wanted a professorship, and a professorship required original research. His hypothesis - that injecting live tuberculosis bacilli would work as a vaccine - had already been disproven, and he layered onto it the pseudoscience of Nazi racial theory, the notion that race determined susceptibility to the disease. Personal connections opened the doors: his uncle was SS General August Heissmeyer, and he was on good terms with SS General Oswald Pohl. He began with Soviet and other adult prisoners at the Neuengamme concentration camp, then decided he needed children. Josef Mengele selected twenty at Auschwitz and sent them north. Four women prisoners travelled with them: two Polish nurses and a Hungarian pharmacist, all three murdered on arrival, and a Polish-Jewish doctor, Paula Trocki, who survived and later testified in Jerusalem about what she had seen. At Neuengamme the children were infected, fell ill, and had lymph nodes surgically removed for study. Each was photographed holding one arm up to display the incision.

The school at Bullenhuser Damm, in the bombed-out district of Rothenburgsort, had stopped being a school in 1943; the neighbourhood around it was rubble. In October 1944 the SS turned it into a subcamp of Neuengamme, housing prisoners sent out to clear the debris. It was evacuated on 11 April 1945, leaving only two SS guards and a janitor behind. Nine days later, with the British Army a fortnight from the city, orders came from Berlin to destroy the evidence of Heissmeyer's experiments. The children were told they were going to Theresienstadt. According to one of the SS men present, they were cheerful on the way, glad to be out of the camp. In the basement of the school they were murdered by hanging, along with the four prisoners who had cared for them: the French physician Rene Quenouille, the Lyon chemistry professor Gabriel Florence, and two Dutch resistance members, Anton Holzel and Dirk Deutekom. Six Soviet prisoners were killed with them, and some two dozen more were brought to the school and murdered later the same night.

Most of those involved were tried by a British military court in Hamburg in 1946. Neuengamme's commandant Max Pauly, the camp doctor Alfred Trzebinski, and the guards Wilhelm Dreimann, Adolf Speck, Ewald Jauch and Johann Frahm were sentenced to death and hanged at Hamelin Prison on 8 October 1946. Heissmeyer simply went home to Magdeburg, in what became East Germany, and built a successful practice as a lung and tuberculosis specialist. He was not identified until 1959, was convicted in 1966, and died in prison the following year. Asked at his trial why he had not used guinea pigs, he replied that for him there was no basic difference between human beings and guinea pigs. Arnold Strippel, who oversaw the killings, never answered for them at all: in 1967 a Hamburg prosecutor dropped the case on the grounds that the children had not been harmed beyond the extinction of their lives. Strippel died a free man in 1994.

Survivors of Neuengamme asked for a plaque in the school in 1959. It took until 1963 to agree on wording, and the wording that emerged said nothing about the children being Jewish, gave no names, and left out the Soviet victims entirely. The real recovery of the story belongs largely to the journalist Gunther Schwarberg, who spent years tracing the twenty children and their families. In 1980 the Senate of Hamburg declared the building a memorial site and named the school for Janusz Korczak, the Polish-Jewish paediatrician murdered at Treblinka alongside the children of his orphanage. A rose garden was planted in 1985, and visitors have been planting roses in it ever since. Streets in the Schnelsen quarter now carry the children's names. Relatives keep arriving: Ruchla Zylberberg's father came, and Sergio De Simone's mother, and Eduard Reichenbaum's brother Itzhak, every year until he died. Walter Jungleib was only identified in 2015, when his elderly sister in Israel recognised his name. A commemoration is held every 20 April.