The person says subpoenas have been issued to Barclays, Citigroup, Deutsche Bank, JPMorgan Chase, HSBC, Royal Bank of Scotland and UBS.
The person spoke on condition of anonymity because the person was not authorized to discuss the matter publicly.
American and British regulators have already fined Barclays $453 million. The bank has admitted that it submitted false information to keep the rate, known as LIBOR, low.
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