The Swiss Federal Intelligence Service has said it will finally open long-sealed files on the notorious Nazi war criminal Josef Mengele, but without saying when.
Mengele fled Europe after World War Two, but for years there have been rumours that he spent time in Switzerland, even though an international warrant was out for his arrest.
Historians have repeatedly requested access to the files, but until now the Swiss authorities have refused.
Mengele was a doctor who served in Germany's Waffen SS. He was posted to the Auschwitz extermination camp in Nazi-occupied Poland, where he selected those to be sent to the gas chambers – an estimated 1.1 million people died, including about a million Jews.
Known as the Angel of Death, he also selected prisoners, primarily children and twins, for sadistic medical experiments, before sending them to their deaths as well.
After the war Mengele, like many high-ranking Nazis, quickly changed both his uniform, and his name.
With the help of his false identity, he was issued Red Cross travel documents at the Swiss consulate in Genoa in northern Italy, and used them to flee to South America.
The Red Cross intended the documents for thousands of people across Europe who had been displaced or made stateless by the war, but Nazis seeking to escape prosecution also managed to acquire them, something for which the Red Cross has subsequently apologised.
So what is Mengele's connection with Switzerland?
Although he fled Europe in 1949, Mengele had a skiing holiday in the Swiss Alps with his son Rolf in 1956. That information has been known since the 1980s.
Officially, after that, he spent the rest of his life in South America.
But Swiss historian Regula Bochsler always wondered whether Mengele returned again, crucially, after an international warrant for his arrest had been issued in 1959.
Bochsler, while researching Switzerland's possible role as a transit country for fleeing Nazis, discovered that in June 1961 the Austrian intelligence service warned the Swiss that Mengele was travelling under an assumed name, and might be on Swiss territory.
Meanwhile, Mengele's wife had rented an apartment in Zurich, and applied for permanent residency.
"There seems to be evidence Mengele was planning a trip to Europe in 1959," the historian told the BBC. "Why did Mrs Mengele rent an apartment in Zurich?"
The apartment was in a modest suburb, and the Mengele family had the wealth for something much fancier. But it was close to the international airport.
Bochsler was able to see Zurich police files which proved that in 1961 the flat was put under surveillance; police even noted Mrs Mengele driving her Volkswagen, accompanied by an unidentified man.
But was it her husband?
Arresting a wanted war criminal, as Mengele was in 1961, would have involved Swiss federal police. In 2019 Bochsler applied to the Swiss Federal Archive to see their files too.
She was refused. The files were sealed until 2071 on national security grounds, and for the protection of the extended family.
Bochsler was neither the first nor the last to be turned down. In 2025, fellow historian Gérard Wettstein tried again. He too was refused.
"It seemed ridiculous," he told the BBC. "As long as they are closed until 2071, it fuels conspiracy, everyone says 'they must have something to hide'."
Wettstein challenged the decision by taking the Swiss authorities to court, an expensive process for which he sought crowdfunding. "We raised 18,000 Swiss francs (£17,000; $23,000) in just a few days."
And that was when the Swiss Federal Intelligence Service finally changed their minds. In a statement this month which suggests full transparency might yet take some time, it said: "The appellant will be granted access to the file, subject to conditions and requirements yet to be defined."
Not everyone is sure the files will reveal much about Mengele himself.
Sacha Zala, president of the Swiss Society for History, is "absolutely sure there is nothing relevant about Mengele", but thinks there may be references to a foreign intelligence service or foreign informants.
By the late 1950s, Israel's Mossad was actively tracking fugitive Nazi war criminals, and Zala suspects they may have been in touch with the Swiss. That would give the Swiss authorities grounds to keep the files sealed, since sensitive information related to foreign intelligence agencies is often redacted.
But is a simple mention of Mossad relating to their well-known hunt for Nazis 70 years ago really so sensitive?
"It shows the stupidity of the declassification process without historical knowledge," Zala believes. "In this way, the administration fueled conspiracy theories."
Other historians, like Jakob Tanner, say the secrecy over the files reveals more about Switzerland than they may ever do about Mengele. "It's a conflict between national security and historical transparency, and the former often prevails in Switzerland."
Tanner served on the 1990s Bergier Commission examining neutral Switzerland's relations with Nazi Germany, in particular the role of Swiss banks.
He is very familiar with Swiss sensitivity, and shame, over its role in World War Two, when Jewish refugees were turned away at the border, while Swiss banks kept the money of Jewish families who later died in Nazi concentration camps. "It is a problem for a democratic state that these files are still closed," Tanner argues.
Still, he thinks it is plausible Mengele was in Switzerland in 1961.
Wanted Nazi war criminal Adolf Eichmann had been arrested by Mossad in Argentina in 1960, and there is evidence that other Nazis who fled to South America thought they too were at risk there, and that Europe, where friends and relatives remained, might be safer.
Tanner points out that Walter Rauff, another wanted Nazi war criminal who fled to Chile, spent time in Germany in 1960.
One historian on the Bergier Commission was permitted, briefly, to look at some of the Mengele files in 1999, and concluded that it was impossible to prove or disprove his presence on Swiss territory. But that was just a few lines in a 24-volume report about the entire war. The files were sealed again; the historian died seven years ago.
Meanwhile, no date for the files' release has been set, and the statement from the Federal Intelligence Service about "conditions and requirements" sounds ominous to Wettstein. "I fear we will get a file that is more black than transparent," he says.
Bochsler also worries the files will be heavily redacted. "I don't trust [the authorities] at all. I fear it will look like the Epstein files. Why have these Mengele files been closed for so long?"
Mengele has been the subject of mystery, rumour and conspiracy for decades.
He was never arrested, let alone convicted for his terrible crimes. When he died in Brazil in 1979, he was buried under a false name.
But rumours continued to swirl. In 1985 his body was exhumed, and finally in 1992 DNA testing confirmed the body was his.
Auschwitz's terrible doctor was dead.
But was he ever in Switzerland? Did the Swiss just not notice him?
Did they turn a blind eye to a potentially embarrassing presence to avoid the unwelcome attention an arrest would have caused? Or, like so much about Mengele, is it all just a rumour?
"Maybe we will never get to the real truth," says Wettstein. "We will never know if he was here or not… but maybe we can have at least a clearer idea."
