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BERLIN — German special police commandos have arrested four men aged 18 to 20 suspected of stealing a gold coin worth about $4 million from Berlin's Bode Museum — but they failed to find the coin.

The arrests came on Wednesday as part of an investigation involving 300 police officers. But officials said they believe the 100 kg (220lb) Canadian "Big Maple Leaf" coin, made with unusually pure gold, has been melted down.

The world's largest gold coin, a 2007 Canadian $1,000,000 Maple Leaf, at the Dorotheum auction house after an auction in Vienna on June 25, 2010. Heinz-Peter Bader / Reuters file

Armed police in balaclavas were outside a property in the Neukoelln area of Berlin, along with paramedics.

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The spectacular robbery from one of Berlin's most prestigious museums in the center of the capital, and from behind bullet-proof glass, stunned Germans.

Police and prosecutors told reporters the four men, aged between 18 and 20 years-old, were linked to an Arab clan suspected of being involved in other crimes such as jewelry theft.

Prosecutor Martina Lamb described how she believed the men had, with the help of someone who worked at the museum, used a ladder, wheelbarrow and rope to extract the coin and escape with it. Police have also said they believed the robbers had broken in through a window at the museum.

Surveillance video dated March 27, 2017, and taken at Hackescher Markt train station reportedly shows three suspects of the robbery of the "Big Maple Leaf" gold coin from the Bode Museum in Berlin. Berlin Police / EPA

Investigators used videoed material to track down the suspects.

The coin, with a face value of $1 million, bears the image of Queen Elizabeth II. It is made out of pure gold with a material value of about $4 million.

"Unfortunately, we assume that it has been at least partly, if not completely, broken down," said Carsten Pfohl of the regional crime office.

The coin, 53 centimeters (20 inches) in diameter and 3 centimeters thick, even made it into the Guinness Book of Records for its unrivalled degree of purity. It was lent to the Bode Museum in December 2010.

The Bode has one of the world's largest coin collections with more than 540,000 items.