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The Westpac Bank has been caught up in what US authorities are labelling the largest international money laundering case ever brought.US prosecutors accuse digital currency exchange Liberty Reserve, which is based in Costa Rica, of helping criminals launder more than $6 billion in criminal proceeds.Liberty Reserve handled huge amounts of money outside the control of national governments.Westpac is one of the many banks around the world that is holding funds from the company, thought to be worth about $40 million.US attorney Preet Bharara says Liberty Reserve facilitated criminal activity.