For investment firms, money-market funds would probably be most affected by Libor-rigging, said Robert Pozen, a senior lecturer at Harvard Business School. Returns earned by investors in money funds, which hold only short-term debt, would decline if Libor were kept lower, he said.

BlackRock, which oversees $3.56 trillion; Fidelity, which manages $1.6 trillion; and Vanguard, with $2.1 trillion, said they're examining the impact on clients.

Evaluating Actions

"On behalf of our clients and shareholders, we have been following developments in the Libor market and the related litigation activity for some time," Vincent Loporchio, a spokesman for Boston-based Fidelity, said in an e-mailed statement. "We have noted recent news with interest and continue to evaluate our options."

BlackRock, the world's largest asset manager, said litigation surrounding Libor is complex and that "it will be some time before greater clarity emerges," according to Bobbie Collins, a spokeswoman for the New York-based company.

Laurence D. Fink, chief executive officer of BlackRock, said in a July 3 interview on Bloomberg Television's "Market Makers" that Diamond "led with a lot of emotion which obviously" angered regulators and others in the U.K. "For me, it's sad. I know Bob very well," Fink said.

Barclays owned a 19.6 percent stake in BlackRock until May, when the bank sold the holding and Diamond stepped down from the money manager's board. Barclays took the stake in December 2009, when it sold its investment unit to BlackRock.

Vanguard, the world's largest mutual-fund company, "will take some time to determine the impact and conduct a cost- benefit analysis before pursuing other actions on behalf of the funds," John Woerth, spokesman for the Valley Forge, Pennsylvania-based firm, said in an e-mail.

'Considering Litigation'

"We are evaluating the facts and considering litigation," Meghan McAndrew, a spokeswoman for Pittsburgh-based Federated Investors Inc., said in a statement. Federated manages about $364 billion and is the third-biggest provider of U.S. money- market funds behind Fidelity and JPMorgan Chase.

It wouldn't be the first time that investment firms would be pitted against banks over losses stemming from the financial crisis. BlackRock and Pacific Investment Management Co. were part of an investor group that brought claims against Bank of America Corp. over mortgage bonds they had purchased, which last year resulted in an $8.5 billion settlement agreement. That settlement is now being challenged by other investors and state regulators.