[ by Melanie Gretchen ] The U.S. is conducting a criminal investigation into suspected manipulation of benchmark interest rates, the Justice Department said in a letter to a federal judge.  The 2/27 letter to U.S. District Judge Naomi Reice Buchwald in Manhattan was made public this week in the first public acknowledgment by the Justice Department of the criminal investigation of benchmark lending rates such as the London interbank offered rate, known as Libor. The Letter. Judge Buchwald referred to the letter at a hearing on 3/1 in which she denied a request for documents related to the investigation by investors suing Credit Suisse, Bank of America, and other companies over claims they artificially suppressed Libor.  According to the letter, the "Department of Justice is conducting a criminal investigation into alleged manipulation of certain benchmark interest rates," including those for "several currencies" on the Libor exchange.  It was signed by lawyers from the fraud section of the Justice Department’s criminal division and its antitrust division. Investigation. Regulators worldwide are investigating whether banks attempted to manipulate Libor and the Tokyo and euro interbank offered rates, known as Tibor and Euribor.  Judge Buchwald is presiding over multidistrict litigation involving 21 class-action, or group, lawsuits against banks accused of conspiring to artificially suppress Libor by understating their borrowing costs to the British Bankers’ Association. The defendant banks objected to the plaintiffs’ demand for documents that have already submitted to the Justice Department, in a 2/17 letter to Judge Buchwald.  "Plaintiffs have demanded discovery to help them draft their consolidated complaints," said lawyers for Bank of America, Credit Suisse, Barclays Bank, HSBC Holdings, JPMorgan Chase, Lloyds Banking Group, Norinchukin Bank, and Royal Bank of Canada. The Justice Department letter doesn’t identify the banks that received subpoenas. The banks said turning over such evidence to plaintiffs would impose a "substantial burden" because they are already complying with requests from "U.S. and foreign government authorities." The Request. In the government’s letter to Judge Buchwald, the Justice Department’s lawyers asked that they be allowed input on "the timing and scope of discovery."  They said they may submit papers to the judge alone "to protect the confidentiality of details of the investigation." Judge Buchwald denied the investors’ request for documents in a ruling from the bench on 3/1.  "To me, it is simply too much to have them piggyback on the government’s investigation at this stage," she said. The case is: In re: LIBOR-Based Financial Instruments Antitrust Litigation, 11-MD-2262, Southern District of New York (Manhattan). For further details, go to [Bloomberg, 3/6/12].