Seven banks, including HSBC and Royal Bank of Scotland, are to be questioned in the United States over suspected manipulation of the Libor inter-bank lending rate.
Barclays, Citigroup, Deutsche Bank, JPMorgan and UBS have also received subpoenas from the attorneys general of New York and Connecticut.
Last month, Barclays was fined £290 million by UK and US regulators for rigging Libor.
US regulators said they were investigating potential involvement by other banks.
Banking analyst Ralph Silva says the investigation is based on the assumption that at least one bank must have colluded with Barclays in any attempt to manipulate Libor rates.
