The Vatican has received a ransom demand for the return of two rare documents, one a handwritten letter of Michelangelo, that have been missing from St. Peter’s basilica since 1997.

Vatican spokesman Father Federico Lombardi confirmed Sunday that Cardinal Angelo Comastri, the archpriest of St. Peter’s basilica, had been contacted recently by a person who offered to retrieve the documents for “a certain price”.

Italian newspaper Il Messaggero said the person, described as a former Vatican employee, had asked for €100,000 for the precious objects. Father Lombardi said the cardinal “naturally refused” to pay the ransom “because these are stolen documents”.

The missing artifacts include possibly the only letter handwritten by Michelangelo Buonarroti preserved at the Vatican. The Tuscan artist was said to be nervous about writing letters himself and so would usually would dictate them to his assistants and sign them afterwards.

It’s not clear what the second document is, but both were taken from the archives of the Vatican department that looks after the upkeep of the basilica, known as the Fabbrica di San Pietro.

The archives, located in rooms adjacent to the basilica, contain documents related to the basilica dating back to 1506 when Pope Julius II created the Fabbrica to follow the construction phase of the basilica and ensure its future maintenance. The archives contain numerous documents concerning the history, design and construction of the basilica. They are not open to the public and few have direct access to them.

Father Lombardi said Vatican police were now investigating the case together with their Italian counterparts. The Vatican has never previously reported the theft.

Michelangelo, who lived from 1475 to 1564, helped design the basilica, painted the ceiling and altar wall of the Sistine Chapel, and sculpted the Pieta.