The Financial Times reports that Eric Schneiderman, New York attorney-general, and George Jepsen, Connecticut attorney-general, sent subpoenas to Bank of America, Bank of Tokyo Mitsubishi UFJ, Credit Suisse, Lloyds Banking Group, Rabobank, Royal Bank of Canada, Société Générale, Norinchukin Bank and West LB.
Banks already embroiled in the investigation include Deutsche Bank, Citigroup, JPMorgan Chase, Royal Bank of Scotland, Barclays, HSBC and UBS.
The FT said the banks all declined to comment, could not be reached for comment or have not responded to requests for comment.
In June Barclays paid US and UK authorities roughly $450m to settle allegations it had attempted to manipulate LIBOR.
Several senior executives were forced to resign as a result including chief executive Bob Diamond.
