The theft of priceless Romanian artefacts including a gold helmet from the Drents Museum in Assen was carried out on the orders of criminals in the southern European country, police believe.

The golden helmet of Coțofenești and three gold bracelets were stolen by thieves who used explosives and hammers to break into the museum overnight on January 24.

Police sources told RTL Nieuws that the thieves were connected to the banned Hardliners biker gang. They believe they were instructed by Romanian underworld contacts who wanted to use the helmet as a bargaining chip to secure the release of gangsters from prison.

Seven people have been arrested, two of whom have appeared in court, but the helmet, which dates from the fifth century BC and has an insurance value of €5.8 million, is still missing.

Police also say the artefacts are unlikely to have been melted down, as was originally feared, because they are more valuable to the gangsters as cultural assets. During the raid the thieves dropped the helmet on the floor, RTL reported.

The investigation has focused around the Noord-Holland town of Heerhugowaard, where five of the suspects are from and where the getaway car used by the robbers was stolen.

“There are growing indications that the helmet has not been melted down,” a source told RTL. “It’s probably still wrapped in a dishcloth somewhere near Heerhugowaard.”