General view of the Dresden castle, which houses the Green Vault museum, in Dresden, Germany, 26 January 2022; Credit: Reuters / Matthias Rietschel

BERLIN (Reuters) - Six German men accused of involvement in a 2019 jewel heist at a museum housing one of Europe's greatest art collections appeared in court in Dresden on Friday, with the whereabouts of the treasures still a mystery.

The defendants, aged between 22 and 28, who were not named under German privacy laws, are charged with aggravated gang theft and serious arson, according to the Dresden public prosecutor's office.

They are suspected of breaking into Dresden's Gruenes Gewoelbe (Green Vault) Museum in the early hours of 25 November 2019, and stealing 21 pieces of jewellery containing more than 4,300 diamonds with an estimated value of more than €113 million.

"The defendants allegedly prepared the crime meticulously", said Thomas Ziegler, a spokesman for Dresden district court.

He said prosecutors believe that the six had checked out the crime scene beforehand, sawn through part of a window grating in advance and reattached it to get into the building as quickly as possible during the heist.

Prosecutors said in September the defendants had not shed any light on the charges against them. Police offered €500,000 as a reward for anyone who could give information on the jewels' whereabouts.

"So far, there is no hot lead", Juergen Schmidt, a spokesperson for Dresden prosecutor's office, told Reuters on Friday, adding that even if convicted, the defendants cannot be forced to give any testimony in court on the whereabouts of the treasures.

There is no evidence so far that the jewellery sets were destroyed or sold, said Anja Priewe, a spokesperson for the Dresden State Art Collections museum.

"We hope that the renewed attention will lead to more insights... and more details will be revealed that will help locate the stolen jewels", Ms Priewe said, adding that similar cases showed that it often takes a long time to solve them.

In 2016, two paintings by Vincent Van Gogh, valued at €50 million each, were recovered more than thirteen years after they were stolen in a Mafia heist.

Four of the suspects could face up to fifteen years in prison if convicted, and the maximum penalty for the youngest defendants, who are twins, would be ten years of juvenile imprisonment, Juergen Schmidt said. All defendants are from the same family, he added.

The trial is expected to continue until the end of October. More than a dozen defence lawyers are representing the suspects who are in custody.

Two have already been sentenced to four and a half years in prison for their involvement in stealing the Big Maple Leaf, a 100-kg gold coin worth €3.75 million, from Berlin's Bode Museum in 2017.

The stolen Dresden collection was assembled in the 18th century by Augustus the Strong, Elector of Saxony and later King of Poland, who commissioned ever more brilliant jewellery as part of his rivalry with France's King Louis XIV.

The treasure survived Allied bombing raids in World War Two, only to be carted off as war booty by the Soviet Union. They were returned to Dresden, the historic capital of the state of Saxony, in 1958.