The question has eluded historians for many decades and is set to remain a mystery for some time. The latest study by the Netherlands Institute for War Documentation (NIOD) has been unable to discover who led the Nazi occupiers to Anne Frank and her family's secret hideout in a canalside house in 1944.

Recent publications had pointed the finger at three suspects: a warehouse worker, the cleaner and a friend of the Frank family. The accusations prompted NIOD researchers to revisit the question of who informed on Anne Frank and seven other Jews hiding in the cramped secret annexe on Prinsengracht as described in Anne's world-famous diary.

No evidence
But the investigation failed to produce any new conclusive evidence either leading to the culprit or supporting the widely held notion that the Frank family had actually been betrayed. Instead, the NIOD report - due to be presented on Friday but posted on the institute's Web site on Thursday – clears all three main suspects, all of whom are now dead.

The study dismisses the oldest theory that warehouse worker Willem van Maaren ratted on the Franks, saying the case had been rightly dropped in the 1960s. Likewise, NIOD researchers found no evidence to suggest that Lena Hartog, the cleaner blamed in a 1998 Anne Frank biography, even knew the secret annexe existed.

Anti-Semite boaster
The third suspect, Dutch Nazi Tonny Ahlers, is portrayed in the report as a cash-strapped anti-Semite and "a boaster who pretended to be much more important than he really was." Ahlers emerged as the chief suspect in a 2002 biography of Anne's father, Otto Frank, the only member of the eight to survive internment in a German concentration camp.

The biography, by British author Carol Ann Lee, suggests Ahlers betrayed the Franks to earn himself a bounty. She also accuses Ahlers of blackmailing Otto after his return to the Netherlands, threatening to reveal Frank's business dealings with Germany during World War Two. But the report found no evidence to support these allegations, said to be based on comments made by the suspect himself. NIOD concludes that "Ahlers was somebody with whom one should be on one's guard, but above all somebody whose word was not to be taken on trust."

A matter of chance
While in hiding, Anne Frank kept a diary for more than two years. Published by Otto Frank in 1947, the Diary has sold millions of copies around the world.

Anne ended up in the Bergen-Belsen concentration camp where she died of typhus at the age of 14 in March 1945, just two weeks before liberation.

The house on Prinsengracht and its annexe have been turned into a museum, frequented by hundreds of thousands of tourists each year.
But if these three suspects didn't lead police to the secret hideaway behind the movable bookcase on that fateful 4 August 1944, who did?

The answer is set to remain a mystery, according to the NIOD report, which stresses that the annexe was, and still is, visible from the outside from many angles. Any casual passer-by or visitor to adjacent buildings could have noticed something suspicious and notified the authorities, it writes.

"It is therefore possible that chance played a much greater role than has been assumed to date, and that there is less of a need to assume a deliberate attempt to deliver the Frank family and the four others into the hands of the Germans."

While in hiding, Anne Frank kept a diary for more than two years. Published by Otto Frank in 1947, the Diary has sold millions of copies around the world.

Anne ended up in the Bergen-Belsen concentration camp where she died of typhus at the age of 14 in March 1945, just two weeks before liberation.

The house on Prinsengracht and its annexe have been turned into a museum, frequented by hundreds of thousands of tourists each year.