A car pulled up outside a narrow brick building in central Amsterdam, around 10 a.m. on August 4, 1944. High up in the Annex of 263 Prinsengracht, eight Jews had been hiding since 1942. Members of the Gestapo emerged from the car and made their way inside, where they arrested Anne Frank, her family and their four friends. Within a year, seven of these eight were dead. Someone must have tipped them off—but who?

Theories about the culprit have percolated for years: a new employee at Otto Frank’s business, for instance, or the wife of one of the employees who helped to hide the family. But a new book by the son of a member of the Dutch resistance claims to have evidence that may bring historians closer to answering the question.

According to Gerald Kremer’s The Backyard of the Secret Annex, the Guardian reports, the Franks were betrayed by Anna “Ans” van Dijk, a Jewish woman executed in 1948 for her collaboration in the capture of 145 people. Ans van Dijk has often been suggested as a leading suspect. But despite independent and police studies, the Anne Frank House museum and research center has not been able to prove or disprove the theory. Kremer’s book may add another piece of evidence to the pile.

Kremer’s father was an acquaintance of Van Dijk in Amsterdam during World War II, where he worked as a caretaker in an office building close to the Franks’ annex. During the Dutch occupation, two floors of the building were turned into a kind of Nazi office for German authorities and the Dutch Nazi organization the NSB. Kremer’s father, the book claims, remembers Van Dijk making frequent visits to the office, where she would make telephone calls.

Despite having been arrested on Easter Sunday 1943, Van Dijk evaded being sent to concentration camps by promising to work for Nazi Intelligence services. She pretended to be a member of the resistance by helping Jews to hide and obtain false papers—then turned them in, including her own brother and his family. It’s estimated that at least 85 and perhaps as many as 700 people died as a result of her collaboration. (Van Dijk was executed in 1948 for her role in these deaths.)

In early August 1944, the book says, Kremer’s father overheard a conversation between Van Dijk and Nazi officials about Prinsengracht, where the Franks were hiding. That same week, the Franks were arrested—while Van Dijk was away in the Hague.

Speaking to the Guardian, a spokesperson for the Anne Frank House said they had investigated Van Dijk in 2016, and found no conclusive evidence. Their most recent study, in late 2016, suggests that the Franks and their friends were discovered by chance, amid an investigation into claims of illegal ration books. “We have not been able to find evidence for this theory, nor for other betrayal theories,” the spokesperson said.

But the book’s publishing house, Lantaam, took a different tack: “We can’t claim that this is 100 percent the answer but we really do think it is a part of the puzzle that may be able to complete the story.”

Millions of people have wondered about this puzzle: The Diary of Anne Frank, first released in 1947, is often described as one of the most important documents of World War II, translated into more than 60 languages and read by adults and children alike. Frank’s account meticulously and movingly detailed the daily lives of the eight people captured that day. Whether Kremer’s book will bring investigators closer to closing this very cold case remains to be seen.