It started while the war was still on. In 1943 the international section of the British Red Cross was asked by Allied headquarters to set up a registration and tracing service for missing people, and on 15 February 1944 it was formalised under Supreme Headquarters Allied Expeditionary Force as the Central Tracing Bureau. Its address kept changing with the fighting: London, then Versailles, then Frankfurt am Main, and finally Bad Arolsen - chosen because it sat centrally among the Allied occupation zones and still had working infrastructure, having escaped the bombing that flattened the cities around it. The International Refugee Organization took over administration on 1 July 1947, and on 1 January 1948 the name changed to International Tracing Service. For decades its director was appointed by the International Committee of the Red Cross and had to be a Swiss citizen, a neutrality clause written into an institution whose subject was anything but neutral. On 20 May 2019 it took its current name, becoming the Arolsen Archives - International Center on Nazi Persecution, and closed out seven decades of being known as the International Tracing Service.

Laid end to end, the original documents run to 26,000 linear metres, backed by 232,710 metres of microfilm and over 106,870 microfiches. The holdings divide into three: incarceration, forced labour, displaced persons. What that means in practice is the paperwork of a bureaucracy that recorded what it was doing - registration cards, transport lists, death records, questionnaires, labour passports, health and social insurance documents, the forms of ordinary administration applied to people being destroyed. Among the files are documents on Anne Frank and Elie Wiesel, which is what most visitors want to hear about, and millions on people nobody has ever heard of, which is what the archive is actually for. Alongside them sit its own working papers: some three million correspondence files, the child search records, and the Central Name Index that unlocks the rest.

For half a century almost none of this was open. The service, backed by the German government, cited German archival law and its hundred-year privacy rule; critics pointed out that an internationally governed body was arguably not subject to German law at all, and some accused it of keeping the Holocaust's paperwork out of reach. All eleven governments on the ITS's international commission had signed the Stockholm Declaration in January 2000 calling for Holocaust-era archives to open, and for six years little changed. In March 2006 the United States Holocaust Memorial Museum stated flatly that the ITS and the ICRC "have consistently refused to cooperate with the International Commission board and have kept the archive closed." That May the commission voted to open the files and to send a full copy to each member state. Ratification took another eighteen months; Greece filed last, and on 28 November 2007 the archive opened. The files began yielding surprises of their own - Associated Press reporters found a carton documenting a Truman-era escapee programme in which the ITS ran background checks on people fleeing Eastern Europe, some of whom the CIA then recruited and sent back. Many of them vanished, presumed dead. In 2013 the archives entered the UNESCO Memory of the World register.

More than a thousand inquiries arrive every month, from all over the world. The mix has changed: searching for living survivors now accounts for around three per cent of the work, and most letters come from grandchildren and great-grandchildren who want to know what happened to someone in their family, often after a death has left them with a name and no story. In 2015 alone the service handled roughly 15,500 requests concerning 21,909 people. It has been overwhelmed before. When the Remembrance, Responsibility and Future foundation began compensating Eastern European forced labourers between 2000 and 2007, some 950,000 enquiries landed on the tracing service, producing a backlog that damaged its standing. The institution now employs around 215 people, funded by the German Federal Foreign Office.

The current project is to make the paper searchable by anyone. Scanned documents have been going online through the archive's digital collections since 2015, and the transcription - turning handwriting and stamped forms into text a database can read - is being done by volunteers on the crowdsourcing platform Zooniverse under the name Every Name Counts. By September 2022 roughly 46 per cent of the archive had been transcribed. It is a strange kind of memorial: thousands of strangers, at kitchen tables, typing out the names of people they never knew so that someone can find them. The town has its own reason to take it seriously. From 1943 to 1945 Bad Arolsen housed a subcamp of Buchenwald holding 185 prisoners - a reminder that the place where the records are kept was not standing outside the history they record.