Citigroup revealed the inquiry in its quarterly corporate filing in the United States, The Wall Street Journal said Tuesday.
The bank said in its Securities and Exchange Commission report that it had "received additional requests for information and document from various domestic and overseas regulators and enforcement agencies, including the Monetary Authority of Singapore and a consortium of state attorneys general."
Citigroup said it was cooperating with all of the investigations.
The bank regulator in Singapore had said in September it would launch an investigation into Libor manipulation, but it did not name the targets of its inquiries.
Libor, which stands for London inter-bank offered rate, is an average of interest rates large banks charge each other for loans. Authorities in June disclosed that British bank Barclays had agreed to a $450 million payment to settle an investigation into Libor manipulation.