Derivatives regulators told the U.S. Justice Department they've found evidence of criminal behavior following an investigation into banks' alleged manipulation of ISDAfix, a benchmark used to set rates for trillions of dollars of financial products.

The U.S. Commodity Futures Trading Commission (CFTC), 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 helping 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.