The United States Justice Department said in a statement late Tuesday that it has seized $3.6 billion worth of Bitcoin stolen during the 2016 Bitfinex exchange hack.

A couple has been arrested for conspiracy to launder cryptocurrency stolen from the exchange, it added.

Nearly 120,000 units of digital currency Bitcoin worth about $72 million back in the day was stolen from the exchange platform Bitfinex in Hong Kong in August 2016, rattling the global Bitcoin community in the second-biggest security breach ever of such an exchange.

But the biggest case was in 2014, when the Tokyo-based Mt Gox trading exchange, then the largest in the world, declared bankruptcy when hundreds of millions of dollars in Bitcoins vanished or were stolen.

The company admitted 850,000 coins – worth around $480 million at the time – had disappeared from its digital vaults.

The collapse left a trail of angry investors calling for answers and denting the virtual currency's reputation.