Regulators probing alleged manipulation of a key interbank lending rate have focused their demands for information and interviews on five global banks, according to people familiar with the investigation.

UBS, Bank of America, Citigroup and Barclays have received subpoenas from US -regulators probing the setting of the London interbank offered rate, or Libor, for US dollars between 2006 and 2008. Investigators have also demanded information from West LB, although the bank said it had not received a subpoena.

Several witnesses have been interviewed by UK and US investigators, who are examining whether rates were manipulated immediately before and during the financial crisis. At the time, some commentators complained that Libor rates did not reflect the real market.