Hundreds of German special police have raided several buildings in Berlin, arresting four suspects over the heist of a 100-kilogram (221lb) Canadian gold coin stolen from one of the city's museums earlier this year.
Heavily-armed masked police arrested the suspects, one wearing a hood over his head, in Berlin's Neukoelln neighbourhood.
Another nine people are being questioned in the case.
All suspects are related to one another and aged between 18 and 20, police said.
The raids of 13 different buildings lasted several hours, but the gold coin was not recovered.
"We assume that the coin was partially or completely sold," Carsten Pfohl of the Berlin state criminal office told reporters at a press conference.
He added that police also confiscated clothes and cars to comb for traces of gold.
The Canadian "Big Maple Leaf" coin, worth several million euro, was stolen from the Bode museum in March.
Police say the three-centimetre (1.2-inch) thick gold coin, with a diameter of 53cm (20.9in), has a face value of one million Canadian dollars (€670,000).
By weight alone, however, it would be worth almost €3.9m at market prices.
Experts think the coin may have been melted down already to cash in on the gold.
Police also searched a jewellery store in the Berlin neighbourhood, saying they had indications the store may have been involved in the possible sale of the gold.
The thieves were most likely tipped off to the existence of the enormous coin by an acquaintance who worked at the museum as a guard, police said.
At least two burglars broke into the museum at night on March 27, using a ladder to climb to a window from elevated railway tracks.
They grabbed the coin, loaded it onto a wheelbarrow and then carted it out of the building and along the tracks across the Spree River before descending into a park on a rope and fleeing in a getaway car.
Police had published footage from surveillance video asking the public for help in finding the thieves.
The coin, which has an image of the Queen on one side and maple leaves on the other, was on loan from a private, unidentified person, dpa reported.
It is one of only five that were made by the Royal Canadian Mint.