Banks including Barclays - already slapped with a record breaking £290m fine for attempting to manipulate the key interest rates - as well as HSBC and bailed out Royal Bank of Scotland were reported to have received subpoenas from New York attorney general Eric Schneiderman and Connecticut attorney general George Jepsen, who are jointly investigating the alleged rigging of Libor.
JPMorgan Chase, UBS, Deutsche and Citigroup are also said to have received requests for co-operation.
All the banks have already admitted they are co-operating with authorities in the US and other parts of the world in the investigation, which has been going on for over a year. The fine on Barclays covered two offences - traders manipulating the rates to help each other and rivals; and during the 2008 banking crisis submitting rates that were lower than they should have been to avoid any impression that the bank was in difficulty.
Bob Diamond, the Barclays chief executive, quit in the wake of the Libor fine as did Jerry del Missier, his close colleague who instructed the bank's Libor submitters to cut their rates during the 2008 banking crisis after misunderstanding a conversation with Diamond.
The bank is still seeking Diamond's successor but last week named Sir David Walker as chairman from 1 November when Marcus Agius will step down.
RBS chief executive Stephen Hester warned last month that the bank would also likely face fines.
The UK has embarked on a review of Libor, which is set when banks submit their estimate of the price they would need to pay to borrow from other banks over periods ranging from overnight to 12 months, in a range of currencies.
Martin Wheatley, the senior Financial Services Authority figure conducting the review, will report back next month on how the interest rate used to price financial products around the world can be reformed.
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