The Swiss Federal Intelligence Service has said it will finally open long-sealed files on the notorious Nazi war criminal Josef Mengele, but without specifying when.

Mengele fled Europe after World War Two, but for years there have been rumors that he spent time in Switzerland, despite an international warrant 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. Posted at Auschwitz, he was responsible for sending many to their deaths—approximately 1.1 million people, including about a million Jews—and conducting grotesque medical experiments.

Known as the 'Angel of Death,' he utilized his medical knowledge for inhumane experiments on prisoners, particularly targeting children and twins.

After the war, Mengele altered his identity and escaped to South America with the help of false documents he obtained, controversially facilitated by the Red Cross.

Although he had a short skiing trip to Switzerland in 1956, historians now are questioning if he may have returned during the time an international warrant for his arrest was in place.

Swiss historian Regula Bochsler, researching the country’s role as a transit point for Nazi fugitives, found indications that Mengele might have attempted to return in 1959. Despite requests for file access over the years being denied initially, there seems to be a growing interest in uncovering the mysteries surrounding his activities in Switzerland.

As the Swiss authorities begin to relent on the release of these files, the historian community looks toward the potential insights that could reshape the legacy of this infamous figure while holding authorities accountable for transparency concerning Nazi war criminals.