In 2015, Flemish journalist Jeroen de Bruyn and Joop van Wijk, Bep Voskuijl's youngest son, wrote a biography, "Bep Voskuijl, het zwijgen voorbij: een biografie van de jongste helper van het Achterhuis" ("Bep Voskuijl, the Silence is Over: A Biography of the Youngest Helper of the Secret Annex"), in which they alleged that Bep's younger sister Nelly (1923–2001) could have betrayed the Frank family. According to the book, Bep's sister Diny and her fiancé Bertus Hulsman recollected Nelly telephoning the Gestapo on the morning of 4 August 1944. Nelly had been critical of Bep and their father, Johannes Voskuijl, for helping the Jews. (Johannes was the one who constructed the bookcase covering the entrance to the hiding place.) Nelly was a Nazi collaborator between the ages of 19 and 23. Karl Silberbauer, the SS officer who made the arrest, was reported to have said that the informer had "the voice of a young woman".