Police have begun trawling a river in Belgium as they try to track down the thieves who stole tens of millions of euros’ worth of jewellery at the Tefaf art fair in Maastricht two years ago.

Dutch and Belgian police are searching the water in Verviers with special equipment, including one metre-wide magnets, in the hope of finding tools used during the raid.

The four robbers, disguised in smart suits, flat caps and glasses, smashed open a display case and brandished guns at visitors as they made of with the jewels. Police have traced the gang to the Serbian city of Niš.

The insurance company covering the event put up a €500,000 reward for information leading to the recovery of the jewels, which include a necklace containing a rare yellow diamond that is estimated to be worth €27 million on its own.

In recent months police have reconstructed the route used by the gang to flee Maastricht in June 2022, including details of addresses where they stayed and the cars they drove.

Their enquiries indicate that some of their equipment may have been abandoned in a stream in the Belgian city.

“There are still a few pieces of the puzzle missing,” the research team leader said in a statement on the police website. “We are hoping to recover items that were used in the robbery. That will be a new piece to add to this larger puzzle.”

In May this year a woman who hired one of the cars used in the raid from a company at Frankfurt Airport was identified by police. Two of the stolen diamonds were recovered at around the same time.