Treasury Secretary Timothy Geithner on Thursday said he did not know whether three major US banks were involved in the manipulation of a key benchmark borrowing rate.

Answering Senate Banking Committee questions about Bank of America, Citibank and JP Morgan Chase's possible roles in the global Libor fixing scandal, Geithner referred lawmakers to investigators.

Asked whether he was aware of possible wrongdoing, Geithner said: "We don't know that. But I think that is a question you need to refer to the enforcement agencies."

"But I think that you are going to find, because this is still a confidential investigation they won't be in a position to answer that question until the remaining investigation is brought to its natural conclusion."

The three banks are on the panel that sets the US dollar interbank lending rate -- which is the benchmark for a range of consumer products and is thought to have been widely abused.

Already Barclays has been fined $452 million by US and British regulators for its role in the scandal.

The US Securities and Exchange Commission and the US Department of Justice are said to be investigating.