During the raid, a policeman emptied Otto's briefcase to fill it with the fugitives' valuables. In his haste, he dropped a batch of papers and a small diary belonging to Otto's daughter. This diary, the diary of Anne Frank, was to become the most widely read document to emerge from the Holocaust.
In March this year, Carol Ann Lee's biography of Otto Frank was published in the Netherlands, generating renewed interest in the diary and reviving the question of who betrayed the Franks. In a television interview, the day before her book was published, Lee identified Tonny Ahlers as that person.
Ahlers was a violent anti-semite. By the early 1940s he had a lengthy criminal record and had been involved in numerous brawls in Jewish-owned cafes. During the war he denounced Jews and members of the Dutch underground to the Germans. In 1945, Ahlers was tried for his wartime activities and sent to prison.
Less than 48 hours after the publication of her book, Lee received an astonishing telephone call from her editor. "Someone rang just now," she told Lee. "He has information about the betrayal of the Frank family. He left his number." Lee called. The man who answered introduced himself as Anton, Tonny Ahlers's son.
"I could never have told people voluntarily that my father betrayed Otto Frank, but now that it has been made public, I feel it's my duty to tell what I know and to prevent any lies and half-truths going into the papers," he explained.
Anton is a reserved man in his mid-50s, who weighs his words carefully. He says he seeks neither fame nor revenge. His Dutch wife had five uncles executed by the Nazis for their resistance activities. He has never been to the Anne Frank house. "I feel shame," he says, "I am ashamed that my father created this situation."
He agrees to meet me, accompanied by Lee, in the lobby of a hotel on the outskirts of Amsterdam. This is the first time he has agreed to be interviewed.
Anton does not remember when he became aware of his father's chequered past. "It was a process, not an incident," he says. "One day, one of the kids at school taunted me, calling me a Nazi boy. Then, when I was 16, I had a girlfriend. I was very keen on her, but when her father found out my identity, he said, 'Not with a Nazi' and forbade her to see me again."
Anton's mother lives in Amsterdam. When Lee approached her in connection with her research on Otto Frank, she got a hostile reception. "I asked her about her ex-husband's relationship with Otto. At first she told me that they were friends and had business relations. But when I confronted her with letters that Ahlers had written about Otto Frank, where it was clear that he hated him, she became aggressive and threatened to call the police. She screamed: 'If you come any closer to this door, I will attack you. The war was bad for everyone, not just the Jews. Otto Frank was my best friend. My husband did nothing wrong during the war. You have no idea what it was like for ordinary Dutch people - everyone talks about the Jews, but it was bad for us too. Anyway, I had Jewish girls working for me during the war, all the time. My husband did not betray anyone. Don't you dare write anything bad about him. If you do, I have family who will come and get you.' And she slammed the door."
Anton is unmoved. "My mother lives in lies. She claims she had Jewish maids working for her during the war but I can categorically say that it's untrue. Lies, lies, lies," he sighs. "My father was a violent man. I remember plates smashing against the walls and punches flying into our faces - my mother saw what was going on but never defended us. She never interfered."
Anton has no doubt that it was his father who denounced the Franks to the Nazis. His evidence is difficult to corroborate, but through painstaking research - poring over letters, listening to testimonies and uncovering wartime documents - Lee has given substance to many of Anton's claims. In her biography, recently published in English, Lee describes Ahlers as an unpleasant and dishonest man. His son concurs: "He was always causing trouble, picking arguments with neighbours and snitching on people. He was always in trouble with the police, always owing money. We often had the bailiffs coming to our flat."
In 1985, the relationship between Ahlers and his wife reached rock bottom. In one incident, she later claimed, Ahlers had tried to run her over with his car. After a particularly violent incident she left him and the following year began divorce proceedings.
At about the same time, Anton's business took a serious blow and he was forced to declare bankruptcy. His lawyer asked him whether he had dealings in the West Indies. "I was rather surprised since I had never been to the West Indies and had no business connections there."
The lawyer showed him a letter sent anonymously to the receivers. The writer claimed that Anton and his wife were involved in drug trafficking in the West Indies. "This letter caused us a lot of problems," he says. "Shortly after this incident, we accompanied my mother to my parents' house, to collect some personal belongings. Lying on my father's desk we found a carbon copy of the 'anonymous letter' sent to the receivers. You see, nothing was beyond him.
"When my daughter was in primary school the class learned about Anne Frank and read from her diary. One day," recalls Anton, "she mentioned that her grandfather had told her that he was involved in the Frank family going into hiding. Then she added that he told her he was also there when they got out."
Assuming that this exchange took place, it is not clear what Ahlers meant, since he was not present when the Franks' hiding place was raided. None the less, there is written evidence indicating that Ahlers knew Otto had gone into hiding and was aware of his hiding place.
In a testimony given to Lee, Ahlers' 82-year-old brother, Cas, said that Ahlers told him that he betrayed the Franks during the war. This can only be submitted as supporting evidence as it is based on hearsay. The incriminating evidence against Ahlers is found by piecing together his wartime activities and contact with Otto Frank. "If you put all the pieces together, it all adds up," says Anton. Then he offers another twist to the tale. "It did not end with the betrayal: I believe that my father blackmailed Otto Frank after the war. My father received money every month - he bragged about it. He would buy lots of presents and go on expensive holidays. He told us it was a disability allowance, because he had polio as a child. But this could not be - the monthly payments were comparable with the salary of a board manager. Then, suddenly, in 1980 his financial situation changed and the spendthrift lifestyle ended." Otto Frank died in 1980.
It is unclear what Otto had to hide and why he would let himself be blackmailed. Lee concedes that the evidence for this theory is circumstantial, but offers a possible motive: "Otto sold products to the Wehrmacht, the German army, in 1940, after the invasion of the Netherlands. Miep Gies, one of Otto's employees - who also helped him go into hiding - confirmed this after the war. Ahlers knew about these dealings. When Otto returned from Auschwitz in 1945, having lost his wife and two daughters, he may have feared his company would be confiscated if his war-time business with Germany became known, although 80% of Dutch firms did business with the Germans during the war, mostly out of fear. You must also take into account," she says, "that when he returned to the Netherlands after the war, Otto was considered an 'enemy national' and his situation was very precarious."
Eva Schloss, whose mother married Otto Frank in 1953, dismisses the blackmail suggestion. "Otto was extremely careful with money. I don't believe for a second that he would allow anyone to blackmail him. My mother and Otto did everything together and there is no way my mother would not know about such a thing."
Ahlers died in 2000, aged 82, and there are now no living witnesses who might be able to unlock the mystery. The Netherlands Institute for War Documentation recently stated that, as a result of the findings published in Lee's book, The Hidden Life of Otto Frank, it is officially reopening the investigation into the betrayal, and investigating the possibility that Otto was blackmailed after the war. It will review old files and testimony in search of new revelations and hope to reach some sort of conclusion by the end of the year.
After more than 50 years of silence, Anton Ahlers is keen to shed light on his father's past and expose the truth. "I have kept silent all these years", he says. "It's now time to tell the truth. Enough of lies. No more lies."