Shelves of copies of Anne Frank’s memoirs in public libraries across Tokyo have been mysteriously defaced, and the Japanese government vowing to find out who was behind the odd crime.

The yet-to-be identified bibliophobe destroyed at least 250 books at 31 libraries, most of them versions of Anne Frank’s Diary of a Young Girl and other publications on the Holocaust, according to the council of public libraries in the capital.

Metropolitan police have yet to determine whether the spate of vandalism was done by an individual or a group.

“Each and every book which comes up under the index of Anne Frank has been damaged at our library,” said Toshihiro Obayashi, a library official in West Tokyo’s Suginami ward.

Dozens of pages from at least 119 books about Anne Frank, including her diary, in 11 public libraries in Suginami, officials said.

Kaori Shiba, the archives director for the central library in Shinjuku ward, said each of the 39 books destroyed at three libraries had 10–20 pages ripped out, leaving them worthless.

In Nakano ward, authorities said 54 books from 5 libraries were found to have torn pages. Most of the books are about Anne Frank but some are about the Auschwitz concentration camp.

The Simon Wiesenthal Center, a global Jewish human rights organization, expressed “shock” at the news and called for an investigation.

“The geographic scope of these incidents strongly suggest an organized effort to denigrate the memory of the most famous of the 1.5 million Jewish children murdered by the Nazis in the World War Two Holocaust,” associate dean Abraham Cooper said in a statement.

Chief Cabinet Secretary Yoshihide Suga said the crime was “unacceptable” and said that police will thoroughly investigate the matter.

The motive behind the vandalism remains a mystery since Japan has no history of Jewish settlement and no real history of anti-Semitism, the BBC reports.

Professor Rotem Kowner, an expert in Japanese history and culture at Israeli’s University of Haifa, told the BBC that Anne Frank’s diary has been exceptionally popular in Japan.

The diary, written by Frank as she and her family hid from the Nazis, was published after her death in a concentration camp at age 15. The book was translated into Japanese in December 1952 and topped the bestseller lists in 1953.

By Maesie Bertumen