The Swiss descendant of one of Adolf Hitler’s doctors has published details of letters showing how he treated the Nazi dictator for voice problems, the NZZ am Sonntag newspaper reports on Sunday.

Hitler was treated multiple times by Carl Otto von Eicken, a German otolaryngologist, over 10 years beginning in 1935, according to the newspaper. The doctor’s letters to a cousin were discovered by Robert Doepgen, von Eicken’s great-great-grandson, who found them when he was researching family files for a school project. Von Eicken died in 1960.

British historian Richard J. Evans, a specialist in German history, verified the authenticity of the unpublished letters, which show Hitler’s fear of serious illness. “If there is something wrong, I have to know about it,” Hitler told the doctor after his first consultation in May 1935, according to the letters.

The letters also showed the importance Hitler placed on his voice, which he used in speeches to rally support for his regime. The letters recount, for example, how he postponed an operation to remove a polyp until after a speech, when von Eicken warned Hitler that he would have to rest his voice after the procedure.

In his letters, von Eicken never questioned that he was dealing with a man whose actions caused the deaths of millions of people in the Holocaust and World War II. Asked by Russian interrogators after the war why he did not kill Hitler, von Eicken said, “I was his doctor, not his killer.”

Hitler committed suicide in a Berlin bunker in 1945, shortly before the end of the war.

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