AMSTERDAM — Hiding from the Gestapo in a secret annex of her father's warehouse in Amsterdam during World War II, Anne Frank heard a little knock on the wall. She could not be sure who or what it was, and it frightened her.
She was right to be scared: Just months later, on Aug. 4, 1944, the police discovered the hideout during a raid and arrested her and seven others living behind a movable bookcase. All but Otto Frank, the diarist's father, and later the editor of The Diary of a Young Girl, perished in Nazi death camps.
Who gave them up has remained a mystery. Now, almost 75 years later, a team of experts led by a retired FBI agent is bringing modern forensic science and criminology to bear in hopes of solving one of history's most famous cold case files.
"We will put special emphasis on new leads," said the retired special agent, Vince Pankoke, 59, who is leading the effort. "We need to verify stories as they come in, and we know that is going to lead to further investigation."
In the search for new leads, he and his team are digitally combing through millions of pages of scanned material from the National Archives in Washington as well as archives in the Netherlands, Germany and Israel.
The use of other modern techniques like forensic accounting, crowd sourcing, behavioral science and testimonial reconstruction may also hold promise of a breakthrough. The team, for example, is carrying out a three-dimensional scan of the original house and using computer models to determine how far sounds might have traveled.
Those techniques may allow them to re-evaluate old evidence — for instance, whether the knock on the wall, described in Anne Frank's diary, was someone telling those hiding that they were being too loud, or whether it could have been a trap.
Such modern and expensive techniques were not available to the Dutch national police when they unsuccessfully investigated the case in 1948 and again in 1963.
Much is known about Anne Frank's life during her two years in hiding, thanks both to her famous diary and the accounts of helpers and friends published after the war. But far less is known about the circumstances surrounding the raid on Aug. 4, 1944.
The raid ended her time in the house on the Prinsengracht and precipitated her long and torturous journey to the Bergen-Belsen concentration camp, where she is believed to have died in February 1945.
In a country from which an estimated 108,000 Jews were deported — of which only an estimated 5,500 returned — the reopening of the case is also part of a larger national conversation.
"She used to be the girl that we protected and now she has become the girl that we betrayed," said Bart van der Boom, an expert on the Nazi occupation and a lecturer at Leiden University. "It's a function of how the Dutch perceive themselves during the occupation."
That perception changed in the 1960s, said van der Boom, when the Dutch started to question the traditional narrative that all Dutch people were victims of the Nazis. The Dutch Resistance Museum in Amsterdam, for example, now features a narrative thread describing the life of a collaborator, as well ones about people who stayed neutral, those who resisted and those who were victimized.
At least 28,000 Jews hid from the Germans during the four-year occupation of the Netherlands, van de Boom said. Of those, roughly a third were caught, the vast majority because of the efforts of a small band of paid of collaborators known in Dutch as "Jodenjagers," or Jew hunters, he said.
"We don't know what happened exactly on that fateful day, and there is something intriguing about an open end in a narrative," said Ronald Leopold, the executive director of the Anne Frank House foundation, which runs the museum and conducts research into her life and death.
"Betrayers did not have the classical image we have of perpetrator, those uniformed faces of death," Leopold said.
The figure of the betrayer is important in the life of Anne Frank because, unlike the police and soldiers that would be responsible for her death, the betrayer was possibly known by the Frank family, and almost certainly was not someone wearing an official uniform.
The list of possible subjects has been growing as researchers have proposed new names and theories. While Wilhelm van Maaren, a warehouse foreman, was the primary focus in both Dutch police investigations, the new investigation is open to all possibilities.
"When Otto Frank returned, in the summer of 1945, he assumed someone gave them up," said Gertjan Broek, a senior historian at the Anne Frank House, which receives 1.3 million visitors a year. "It's always been a firmly held belief."
But while the idea that the police were tipped off has long been part of the Anne Frank story, not everyone is convinced that betrayal necessarily played a role.
Broek published a 37-page report in December proposing the theory that the police were at the address on another mission, and found the lodgers only by chance.
Researchers in the Netherlands have welcomed the new investigation, and Broek is serving as one of its advisers.
"What is new about this one is that it looks at the case with forensic eyes," Leopold said. "And we look forward to the results."
The investigative team hopes to reveal its progress on Aug. 4, 2019, exactly 75 years after the raid.