The U.S. Commodity Futures Trading Commission, which first sent subpoenas to the world's largest banks in November 2012 to determine whether ISDAfix was rigged, has flagged its findings to prosecutors, according to a person familiar with the matter. The CFTC's enforcement powers are confined to bringing civil, not criminal, cases. It isn't clear who the CFTC suspects broke the law.

Benchmarks like ISDAfix, which is used to track prices on interest-rate swaps, serve as the foundation of global finance, helping lenders decide how much to charge borrowers and pensions plan their future obligations. Regulators around the world are probing allegations that measures used to set prices in gold, oil, interest rates and currencies were rigged by banks and brokers wanting to pad their profits while cheating their clients and other investors.

Last week, the Alaska Electrical Pension Fund accused 13 banks including Barclays Plc, Bank of America Corp. and Citigroup Inc. as well as broker ICAP Plc of conspiring to manipulate ISDAfix. The U.K. Financial Conduct Authority is also looking into allegations of wrongdoing involving the benchmark.