Police have opened an investigation into two notebooks written by 19th century scientist Charles Darwin that went missing from Cambridge University’s library 20 years ago, according to the Associated Press. The books were only recently reported stolen after Cambridge librarians determined they were not simply misplaced. “My predecessors genuinely believed that what had happened was that these had been mis-shelved or misfiled,” university librarian and director of library services Jessica Gardner said. “Now we have completely reviewed as a new team what happened and come to a conclusion that that’s not a sufficient position or set of actions to take.” The two notebooks include Darwin’s renowned “Tree of Life” sketch and are estimated to be worth millions of pounds. The manuscripts have been missing since 2000, and university staff have searched all 189 boxes of the Darwin archive without any success. The library in total has more than 130 miles of shelving and 10 million books and objects.