Amsterdam, Netherlands ? The enduring mystery of the Anne Frank story is, who betrayed her to the Nazis?

Anne and her family hid for 25 months in a canal-side warehouse in central Amsterdam, where the teen-ager wrote her thoughts, yearnings and descriptions of life in the cramped annex into notebooks.

ANNE FRANK, shown in an undated file photo, kept a diary while her family hid from the Nazis in Amsterdam, Netherlands, during World War II. Biographer Carol Ann Lee has published a new theory saying a former business associate of Frank's father tipped off the Franks' hideout to the Nazis.

First published in English in 1952 as “The Diary of a Young Girl” and later as a stage play and film, her story made her a symbol both of the Holocaust and of Dutch bravery.

On a warm summer day in 1944, four German and Dutch security police pulled up to the warehouse at 263 Prinsengracht and asked the employee, Willem Van Maaren, where the Jews were hiding. Van Maaren pointed up the stairs, but the police already seemed to know exactly where to go.

Hours earlier, Karl Josef Silberbauer, the Austrian commander of the squad, received a phone call from the head of the Amsterdam security police who said eight Jews were hiding in the warehouse.

Who tipped them off?

For more than 20 years, employee Van Maaren was the main suspect. A petty thief and unsavory braggart, Van Maaren was investigated shortly after the war, but nothing was proved. The case was reopened in 1963 after Austrian Nazi hunter Simon Wiesenthal located Silberbauer in the Vienna police force. But the evidence against Van Maaren was again inconclusive, and he died in 1971 professing his innocence. Historians made no further headway.

Now, a biographer of Anne Frank has published a new theory which has intrigued the nation and revived a dark chapter in Dutch history the failure to protect Jewish citizens from the genocidal Nazis.

More than 100,000 Dutch Jews 70 percent of the community, and proportionally more than in any other European country were deported to concentration camps in Germany. Most were gassed with brutal assembly-line efficiency. Anne died of typhus in Bergen-Belsen in the spring of 1945, just weeks before the camp was liberated.

The book by Carol Ann Lee, a British author living in Amsterdam, says the likely informant was a former business associate of Otto Frank, Anne’s father and the only family member who survived the war. His name was Anton Ahlers.

“I looked at his files in The Hague because after the war he was convicted of betraying people and he was jailed,” Lee said in an interview for a Dutch television documentary. “Everyone, including his own family, condemned him as distinctly anti-Jewish and a thoroughly unpleasant character.”

The Netherlands Institute for War Documentation, the researchers who published the authoritative version the diary and the caretaker of the Frank papers, said Ahlers had not been a suspect until Lee started probing his background for her book, “The Hidden Life of Otto Frank,” published in March.

But Barnouw said Lee’s case sounded credible enough for the institute to reopen its investigation into the betrayal. “We are interested, that’s for sure,” said David Barnouw, a researcher and spokesman of the government-financed historical institute. Lee “has no proof, but I can imagine this was the case.”

Barnouw told The Associated Press he and another colleague will review old files and testimony for new revelations. “Sometimes you can go through the same material with fresh eyes.”

Patricia Bosboom, of the Anne Frank Foundation which maintains the house where Anne hid, said Lee’s theory was plausible, “but it’s still only a theory. The proof is not final. It probably never will be. It’s been such a long time, and most of the people who knew are dead.”

Lee says Ahlers not only turned in the Frank family, but may have blackmailed Otto Frank for years after the war, receiving payment for his silence about Frank’s business with Nazi Germany at the beginning of World War II.

The German-born Frank, who moved to Holland in 1933, ran a spice-trading company that sold goods to the Wehrmacht, the German army. The business continued to operate while the Franks were in hiding, although it apparently was no longer trading with the Germans.

When Frank returned from Auschwitz in 1945, having lost his wife and two daughters, he may have feared his company would be confiscated if his prewar business with Germany became known.

Lee says among the four men who raided the Prinsengracht warehouse on Aug. 4, 1944, was Maarten Kuiper, a Dutch policeman who was a friend of Ahlers.

“I think he actually made the call. I think he got the information from Ahlers,” Lee said. “They were friends. Ahlers had so much information on Otto Frank. Maarten Kuiper was one of the major betrayers of Jews in hiding during that time.”

Lee said Ahlers probably decided to tip off the authorities after his own company slid into bankruptcy and he no longer needed to do business with Frank’s company.

“Otto Frank was of no more use to him in that sense, so he betrayed them,” Lee said in the television interview. “He may have got money for it. Certainly, Maarten Kuiper received money for the betrayals he made.”

The Germans were paying a bounty of 40 guilders per head, which was “a large amount in those days,” she said. Anne, her sister and parents, were caught in the two-story annex with Hermann van Pels, who had worked in Frank’s company, with van Pels’ wife and teen-age son, and with Friedrich Pfeffer, a dentist.

After Lee’s book was published, Ahlers’ son was quoted as saying he was convinced her theory is true.

“There’s no doubt he did it,” Anton Ahlers Jr. told the Volkskrant newspaper. Ahlers said he believed his father received money from Frank, because the flow of funds stopped when Frank died in 1980.

Barnouw, the historian, said the blackmail supposition is thin. “There’s no smoking gun, and the theory has too many loose ends,” he said. He also was distrustful of the Ahlers family, saying they simply may be seeking notoriety.

Nevertheless, Lee’s book “is interesting because it takes a more balanced view of Otto Frank,” Barnouw said. “After the play and the book, Otto Frank was kind of like a saint. In this book, he’s much more flesh and blood.”