Fifteen of the world's largest banks are under investigation on suspicion of rigging the Brazilian currency, antitrust watchdog Cade said on Thursday, the first such probe in one of the busiest foreign exchange markets globally.

In a document, Cade alleged that the banks colluded to influence benchmark currency rates in Brazil by aligning positions and pushing transactions in a way that deterred competitors from the market between 2007 and 2013, at least. Foreign exchange trading in Brazil is estimated at about $3 trillion a year, excluding swaps and derivative transactions.

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