The Nazi “Final Solution to the Jewish Question” turned out to not be “final.” A million European Jews survived. Persecuted and pursued, many Jews tried to hide. The Nazis hunted them relentlessly. Still, the Nazis were short-handed. They called upon all sorts of auxiliaries. Sometimes these auxiliaries hoped for a material reward. Other times, they were coerced. Still other times, the Nazis allowed people to voice personal animosities that had not been able to express before. So Jews in hiding found themselves vulnerable to betrayal.
Otto Frank (1889-1980) grew up in a prosperous, assimilated German Jewish family. Other branches of the family settled in Switzerland and France, while Otto spent several years in the United States. He fought in the Germany Army during the First World War, then worked at the family bank. He married in 1925 and fathered two daughters–Margot and Anne. Even before the Nazis came to power in January 1933 the family had considered emigration. In 1934 they moved to the Netherlands. Started over at age 45 as a businessman in a foreign country with a different language, Otto Frank was a resilient man.
In May 1940, the German conquest of the Netherlands renewed the danger to the Frank family. Otto Frank transferred ownership of his business to his employees to forestall “Aryanization” (confiscation). In July 1942, when deportations from the Netherlands began, he took his family into hiding. There they remained until 4 August 1944. Then German police burst in. The Frank family soon went to Auschwitz. Only Otto Frank survived the war.
How had the Germans discovered the Frank family? It could have been an accident, incidental to a search of the building on some other matter like black market food trading. It could have been the result of betrayal. A 1948 Dutch police investigation focused on finding the Germans and the Dutch police responsible for the raid. One of the Germans had hanged himself in 1945 to dodge trial. The other, Karl Silberbauer, had disappeared.
Later, interest turned to an informer. In 1963, Simon Wiesenthal caught up with Silberbauer. Almost twenty years on, the German claimed to have a vivid memory of the arrest itself, but he could say nothing about who had informed. His boss had received a tip.
At one time, suspicion fell on a sister of one of those hiding the Frank family. The sister was said to have had a lover in the German forces and to have worked for the Germans during the Occupation. Purportedly, she had been talking on the telephone in German on the morning of the arrest; and the German who ordered the raid is said to have recalled receiving a tip in a call from a young woman.
A more recent investigation has nominated Arnold van den Bergh (1886-1950). Van den Bergh also was Jewish. It is conjectured that he betrayed the Franks to save his own family. (In 1945, he had been anonymously denounced to Otto Frank.) Not likely scoffed the experts.
In theory, only six people knew of the hiding place. Benjamin Franklin once said that “two may keep a secret if one of them is dead.” Who knows how widely the “secret” spread?
 For an example, see Peter Wyden, Stella: One Woman’s True Tale of Evil, Betrayal, and Survival in Hitler’s Germany (1992). Not that I recommend the book itself, but if you unearth her story it is fascinating.
 See: Robert Gellately, “The Gestapo and German Society: Political Denunciation in the Gestapo Case Files,” The Journal of Modern History Vol. 60, No. 4, December 1988.
 Rosemary Sullivan, The Betrayal of Anne Frank: A Cold Case Investigation (2022).