U.S. regulators have asked Citigroup , Deutsche Bank , Bank of America and JPMorgan Chase to make employees available as witnesses in a Libor probe, Bloomberg said, citing two people briefed on the plans.

The banks, members of the panel that sets the interbank offered rate known as Libor, were asked to appear voluntarily for the interviews in London in April with the UK Financial Services Authority, the people told the agency.

Investigators are probing whether banks understated Libor to reduce borrowing costs and downplay investor panic during the financial crisis.

About $350 trillion worth of financial products globally reference Libor, and lower levels for the rate could have robbed lenders and investors of significant amounts of interest income. Borrowers, on the other hand, would have benefited.

All four banks declined to comment on the matter to Bloomberg.

Reuters could not reach the banks for comment outside regular U.S. business hours.

(Reporting by Sakthi Prasad and Arup Roychoudhury; Editing by Vinu Pilakkott)

Citigroup Deutsche Bank