Hundreds of antique books, including early editions of works by Isaac Newton and Robert Boyle, have been stolen from Cambridge University and the London Library.
Peter Fox, librarian at Cambridge, said the theft at his library was first discovered in 1993. "A good deal of works by Newton, Kepler and Galileo were taken. In all about 100 antique books were stolen," he said.
London and Cambridge police teams have been able to locate and return some of the books to the two libraries.
Sergeant George Barr of Cambridgeshire constabulary said: "The investigation is continuing and a person is assisting with inquiries. In addition to books that have been recovered so far and identified as having been stolen from Cambridge University and London Library, there are a number of others whose ownership has not yet been established."
Mr Fox said the books stolen from Cambridge would have been difficult to sell. "The library community and the book trade work very closely together, and such works coming on the market would be noticed and talked about," he said.
Mr Fox said security at the university's library had been considered good at the time of the theft, but had since been considerably increased. "We obviously cannot say it will never happen, but we have taken all the precautions we can within reason. University librarians store and maintain books not for their value but for their use. It is very important that they are not made inaccessible to students and staff wanting to do serious work on them."
From the London Library, an independent, charitable institution established in 1841, the thief made off with works dealing mainly with 19th-century economic and social history. A large collection of 16th-century German reformation pamphlets was also stolen.
London Library librarian Alan Bell said: "Some of the German reformation pamphlets have been located in Germany, and police are working with auction houses where they were sold in good faith. Items are being returned by police, but we cannot say whether we will eventually recover all the works stolen."