BlackRock Inc., Fidelity Investments and Vanguard Group Inc., firms that collectively manage more than $7 trillion, are gauging how their clients have been hurt by Libor manipulation and whether to take legal action as at least a dozen banks are being investigated for rate-rigging.

The money managers can take cues from Charles Schwab Corp. and the city of Baltimore, which in lawsuits predating the record fine levied on London-based Barclays Plc last month, sued lenders for artificially suppressing, Libor, or the London interbank offered rate. Schwab alleged last year that returns on money funds and short-term debt strategies were depressed by the banks' actions, while Baltimore's lawsuit against Barclays and other banks stems from lower returns on interest-rate swaps.

Libor-related litigation "has the potential to be the biggest single set of cases coming out of the financial crisis because Libor is built into so many transactions and Libor is so central to so many contracts," said John Coates, a professor of law and economics at Harvard Law School in Cambridge, Massachusetts. "It's like saying reports about the inflation rate were wrong."