U.S. authorities are investigating the involvement of seven large banks in the alleged manipulation of the London inter-bank offer rate (LIBOR).

Subpoenas were issued to Barclays PLC (LON:BARC) (NYSE:BCS), Citigroup Inc. (NYSE:C), Deutsche Bank AG (ETR:DBK) (FRA:DBK) (NYSE:DB), JPMorgan Chase & Co. (NYSE:JPM), HSBC Holdings plc (LON:HSBA) (NYSE:HBC), Royal Bank of Scotland Group plc (NYSE:RBS), and UBS AG (NYSE:UBS) by the office of New York Attorney General Eric Schneiderman and Connecticut Attorney General George Jepsen, who are leading the investigation.

The United States government is conducting a criminal investigation regarding the possible involvement of other banks in the LIBOR scandal. Investigators want to find out if there is a conspiracy between Barclays other banks in manipulating the LIBOR rates, and if there’s enough evidences and testimonies to file criminal charges.

Ralph Silva, a banking analyst at SRN said it is the first time for authorities to investigate a rate rigging conspiracy. He said, “This is the first time we’re seeing a legal case that is trying to prove [collusion]. If they can prove it, then all the fees could amount to tens of millions of pounds.”

According to analysts, the civil case filed by investors against the banks seeking damages in losses from the LIBOR manipulation will be strengthened if U.S. investigators file criminal charges. Berkshire Bank, a lender based in New York filed a lawsuit against 21 banks in connection with the alleged LIBOR manipulation, and it is seeking the right to represent other financial institutions in a potential class action lawsuit.

Barclays paid a $454 million fine, after admitting the bank submitted false London and euro interbank offered rates. The bank’s top two executives, Marcus Agius and Bob Diamond, resigned from their position as chairman and chief executive officer respectively.

Martin Wheatley, managing director of the Financial Services Authority said the LIBOR system is no longer a “viable option” after reviewing the process on how the benchmark interest rates for financial contracts are calculated, based on the estimated interest rates on loan in 10 currencies submitted by a group of major banks.

The FSA said, Derivatives traders requested the false submissions in the LIBOR and EURIBOR setting process, as they were “motivated by profit and sought to benefit Barclays’ trading positions.”

According to the report from Bloomberg, RBS and UBS confirmed receiving the subpoena and requests for information regarding the LIBOR scandal. Both banks are cooperating in the LIBOR investigation, according to the spokespersons.

A regulatory filing from Citigroup showed the bank’s subsidiaries continue to receive requests for information regarding the case, from government agencies in the United States, including the offices of the attorneys general in New York and Connecticut. Citigroup also received requests for information from authorities in other countries.

Spokespersons from Barclays, HSBC, JP Morgan, and UBS declined to provide any comment regarding the subpoena, according to the report.

Florida Attorney General, Pam Bondi, is also actively reviewing the issues related to the LIBOR, and issued subpoenas, while Massachusetts Attorney General, Martha Coakley, confirmed last month that her office is working with other state agencies in conducting a LIBOR investigation.