The BBC reported the New York attorney general has also issued subpoenas to Barclays, Citigroup, Deutsche Bank JP Morgan and UBS.
Barclays was hit with the largest-ever fine by the Financial Services Authority (FSA), alongside a huge penalty by the US authorities after it breached rules regarding LIBOR.
The FSA fined the bank £59.5m for significant failings in relation to LIBOR and EURIBOR, after it sought to profit from the settlement process.
RBS has already admitted it is being investigated for fixing Libor, the report added.
It said: “RBS Group continues to receive requests from various regulators investigating the setting of libor and other interest rates.
“We are cooperating with the investigations and are keeping relevant regulators informed.”
