Vatican police have detained a former employee who allegedly tried to sell a 17th-century gilded manuscript describing Bernini’s designs for the altar canopy of St Peter’s Basilica back to the Holy See.

Vatican prosecutors said the 18-page manuscript, which apparently contains the first known specifications for the gilding of the baldacchino canopy, had disappeared from the basilica archives.

The person implicated worked for the Fabbrica di San Pietro, the entity that administers the basilica, and was attempting to sell the manuscript to the Fabbrica when he was arrested on May 27, according to a statement from prosecutors released by the Holy See press office.

The statement suggested that Vatican law enforcement had essentially set a trap. They launched an investigation after the basilica made a complaint about the manuscript, and then followed the negotiations for the purchase of it until money exchanged hands on May 27 in the Vatican.

The former employee has since been in Vatican detention.

Italy’s Domani newspaper, which reported on the case on Thursday, said the provenance of the manuscript and whether it had previously been in the basilica archives is in dispute.

It quoted an art historian, a friend of the accused, who said there is no trace of the manuscript in the catalogue of basilica archives since at least 1900.

The 10-storey high baldacchino is the four-pillared canopy covering the main altar of the basilica, which was designed by Gian Lorenzo Bernini in the 1620s-30s to provide a monumental and ceremonial covering for the tomb of St Peter.

It is considered one of the most complicated multi-material artworks of its time, featuring marble, bronze, wood, gold and iron, and featuring gilded cherubs and laurel branches twisting around the nine-ton columns.

The canopy is currently shrouded in scaffolding for a months-long cleaning and restoration.