Copies of Anne Frank's The Diary of a Young Girl and other books about the Holocaust victim have been vandalized in Tokyo public libraries since the end of January.

Librarians at 31 public libraries have counted at least 265 damaged books with dozens of their pages ripped out.

Japanese police are currently investigating the incidents.

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Japan and Nazi Germany were allies in World War II, and though Holocaust denial has occurred in Japan at times, the motive for damaging the books is unclear.

Chief Cabinet Secretary Yoshihide Suga called the vandalism “shameful” and said Japan would not tolerate such acts.

In the Nakano district libraries, the vandals reportedly damaged the books while inside reading rooms but were not seen, according to city official Mitsujiro Ikeda.

“Books related to Ms. Anne Frank are clearly targeted, and it's happening across Tokyo,” he said Friday. “It's outrageous.”

At least one library has moved Anne Frank-related books behind the counter for protection, though they can still be checked out.

Anne Frank wrote her diary over the two years she and her family hid in a concealed apartment in Nazi-occupied Netherlands during World War II. After her family was betrayed and deported, she died in a German concentration camp at age 15 in 1945.

Her father survived and published her diary, which has become the most widely read document to emerge from the Holocaust.

The Simon Wiesenthal Center, a US-based Jewish human rights organization, issued a statement calling the vandalism a hate campaign and urging authorities to step up efforts to find those responsible.

Additional reporting by AP