Revered Helmet of Coțofenești among items from ancient Dacian civilisation stolen while on loan at Drents Museum

Hours before the sun rose over the Netherlands, the group crowded around the large external door, appearing to pry it open. Seconds later, the grainy security video appeared to show a powerful explosion, sending plumes of smoke and sparks into the air, and the thieves rush into the museum in the north-eastern city of Assen.

Minutes later they were gone. But the mystery of what exactly took place during their few minutes in the Drents Museum – and what came afterwards – has left officials in the Netherlands scrambling for answers, and prompted a row that has stretched to the other side of Europe.