Societe Generale, Royal Bank of Canada and Bank of America are among nine additional banks that were subpoenaed in New York and Connecticut's probe of alleged manipulation of Libor, a person familiar with the matter said.
The subpoenas, issued by New York attorney general Eric Schneiderman starting in August, brought to 16 the number of banks that had been subpoenaed in the states' investigation, said the person, who asked not to be identified.
Schneiderman and Connecticut attorney general George Jepsen are jointly investigating claims that banks rigged the London interbank offered rate, a worldwide benchmark for borrowing.
The same person said in August that seven banks were subpoenaed in their investigation, including JP Morgan Chase and Barclays.
The states are co-ordinating with "a much larger group of attorneys general", Susan Kinsman, a spokeswoman for the Connecticut attorney general, said last month.
Florida attorney general Pam Bondi has also issued subpoenas to more than a dozen financial institutions, including UBS, Deutsche Bank and HSBC Holdings.
Besides Societe Generale, Royal Bank of Canada and Bank of America, the other six banks to receive subpoenas from New York are Credit Suisse Group, Bank of Tokyo Mitsubishi UFJ, Norinchukin Bank, Rabobank, Lloyd's Banking Group, and WestLB, a German lender that has ceased operations.
Representatives of those banks could not be reached for comment or declined to comment on the subpoenas.
Lloyd's has said it is assisting various regulators in their investigations into the setting of Libor. Royal Bank of Canada has said it is co-operating with regulators and found no evidence of collusion with other banks.
Rabobank said in a report this year that it received subpoenas and requests for information from regulatory agencies in the US, Europe and Asia and was co-operating.