Germany\'s biggest bank is reportedly cooperating with European investigators probing the manipulation of interbank interest rates. The bank seeks to limit the damage as regulators have started imposing penalties. Deutsche Bank has asked the European Commission and Swiss authorities to give it the status of cooperating witness in an ongoing investigation of the rigging of the London Interbank Offered Rate (Libor), media reports said on Sunday. According to the German weekly news magazine Der Spiegel, Germany\'s biggest bank had already offered investigators a pact in 2011, in efforts to \"limit damage to its reputation and bottom line.\" \"Anxiety currently reigns within Deutsche Bank,\" a source close to the bank told the German weekly. Deutsche Bank is one of some 20 banks alleged to have manipulated Libor - the key interest rate for daily interbank lending transactions, which include mortgages, loans and credit cards. Libor is compiled daily by the British Bankers\' Association from information provided by major banks on how much they assume they need to pay for borrowing from each other. The banks stand accused of having lied on their input for Libor between 2005 and 2011. Crown witnesses Spiegel said that the European Commission had already granted British bank Barclays as well as Swiss bank UBS the status of cooperating witnesses. In addition, the news magazine reported that the EU executive body had decided to grant total immunity to the first bank to cooperate and lighter penalties for a maximum of two other banks coming clean on their involvement in the scandal. Barclays, the bank said to be at the center of the scandal, was fined a record 350 million euros ($450 million) last month by US and British authorities. According to a study by US investment bank Morgan Stanley, quoted by the DPA news agency Monday, Deutsche Bank\'s exposure might amount to more than $1 billion, with the banks under investigation facing a total of $22 billion in penalties.