(Bloomberg) -- Hong Kong authorities arrested four men for a suspected money-laundering syndicate involving HK$1.2 billion ($155 million) with virtual currency.

Operation “Coin Breaker” was launched on July 8, and the men aged between 24 and 33 have been arrested, according to a statement from Hong Kong Customs. The men opened various local bank accounts and made transactions through a virtual currency exchange trading platform, the statement said. The suspicious funds were processed via bank remittances and a virtual currency during February last year through this May, according to officials.

In the statement, Hong Kong Customs did not name the coin, trading platform or the banks involved. A media briefing is scheduled for 11:00 a.m. local time.

This was the first time Hong Kong Customs detected a money laundering case with cryptocurrency, its statement said.

The former British colony has tightened its oversight on cryptocurrency trading and requires all platforms to register with a local watchdog, and be subject to anti-money laundering and counter-terrorism financing rules. They can only serve professional investors not retail traders, according to a government announcement in late May.

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