French police are searching for four suspects believed to have stolen royal jewellery of “inestimable” historical value from Paris’s Louvre museum, in a heist that forced one of the world’s most famous attractions to close for the day.

The suspects were being sought by police after attempting to make off with nine pieces of priceless royal jewellery, Paris prosecutor Laure Beccuau said on Sunday night. Only eight were taken however, after they dropped the crown of Empress Eugenie, the wife of Napoleon III, as they made their escape. The other items taken include necklaces, brooches and diadems.

The four had their faces covered and threatened security guards with the same tools they used to smash the display cases, before fleeing on high-powered scooters, Beccuau added. Some 60 investigators are working on the case with “total determination” to find the robbers and recover the items.