BRUSSELS (Reuters)—Several banks under investigation for suspected rigging of euro interest rates are cooperating with E.U. antitrust regulators in the hope of lower fines, putting them at a higher risk of lawsuits, two people familiar with the matter said on Monday.

The move by the banks to voluntarily disclose more about their knowledge of possible manipulation of the Euro Interbank Offered Rate, is effectively an admission of wrongdoing and illustrates growing nervousness that they, like Barclays, face a heavy penalty.

The European Commission is investigating possible manipulation of Euribor, the benchmark used when pricing bank lending in euros.

The E.U. watchdog has not disclosed the names of the banks being investigated which could face fines of up to 10% of their global revenues if found to have breached E.U. antitrust rules.

This month, sources told Reuters that Deutsche Bank was already cooperating with the authorities. The lender had revenues last year of €33.2 billion ($40.9 billion).

"Several banks have come forward with information to the Commission," said one of the sources, who declined to be identified because of the sensitivity of the matter. This person declined to provide more details.

The second person said there could be at least two banks, besides Deutsche Bank, which have sought leniency under the European Commission's scheme to encourage whistleblowers.

The E.U. Commission spokesman for competition policy, Antoine Colombani, declined to comment.

Leniency applicants, who in effect are admitting wrongdoing, could find themselves targeted by disgruntled investors, fund managers and investment funds, said Morten Nissen, a partner at law firm Bird & Bird.

"One of the things to take into consideration when applying for leniency is the increased risks of damages actions down the road," he said.

"The facts will be described in the regulator's decision, that is where the risks are," Mr. Nissen said.

Barclays, Deutsche Bank, Citibank, Lloyds, HSBC, JP Morgan and RBS are some of the banks that are the subjects of lawsuits in the United States tied to Libor.

A total of 43 banks sit on the Euribor panel, which is hosted by the European Banking Federation. The rate is used as a reference for trillions of euros in euro-denominated loans and debt instruments.

Under the Commission's leniency policy, the whistle-blower gets off scot-free. Fines can be reduced by 30 to 50% for the next company to provide evidence of wrongdoing, and by 20 to 30% for the following applicant. Subsequent applicants can get a reduction in any penalty of up to 20%.

To qualify, companies must provide what the regulator terms "significant added value" information.

The Commission is also investigating possible manipulation of the London Interbank Offered Rate and the Tokyo Interbank Offered Rate.

Manipulation of Libor, which is used to set prices for trillions of dollars of financial products around the globe, has already landed Barclays with $453 million in penalties from U.S. and British regulators, and cost chief executive Bob Diamond his job. The rate-fixing scandal threatens to drag in several other banks.

Regulators in the United States, Japan and Singapore are also investigating various interest rate benchmarks.

Thomson Reuters Corp is the British Bankers' Assn.’s official agent for the daily calculation and publishing of Libor.