LONDON (AP) — Cambridge University launched an appeal Tuesday to find two valuable notebooks written by Charles Darwin after they were reported as stolen from the university’s library.

The notebooks, estimated to be worth millions of pounds, include the 19th-century scientist’s famous “Tree of Life” sketch. They haven’t been seen since 2000, and for years staff at the library believed that the manuscripts had probably been misplaced in the vast archives.

But after doing a thorough search, library staff now conclude it’s likely that the notebooks were stolen. Police are now investigating and Interpol has also been notified.

“My predecessors genuinely believed that what had happened was that these had been mis-shelved or misfiled,” said Jessica Gardner, university librarian and director of library services. “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.”

Staff recently searched through 189 boxes making up the Darwin Archive, but failed to locate the notebooks.

Cambridge University Library has more than 130 miles of shelving and has around 10 million books, maps, manuscripts and other objects.