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.

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