The U.S. Justice Department is conducting a criminal probe into whether the world’s biggest banks manipulated a global benchmark rate that is at the heart of a wide range of loans and derivatives, from trillions of dollars of mortgages and bonds to interest rate swaps, Reuters reported.

Though the Justice Department’s inquiry into the setting of the London interbank offered rate, or Libor, was known, the criminal aspect of the probe was not, Reuters said.

A criminal inquiry underscores the serious nature of a worldwide investigation that includes regulators and law-enforcement agencies in the United States, Japan, Canada and the United Kingdom.

Several major global banks, including Citigroup Inc, HSBC Holdings plc, Royal Bank of Scotland Group plc and UBS AG, have disclosed that they have been approached by authorities investigating how Libor is set.

No bank or trader has been criminally charged in the Libor probes. It wasn’t clear which banks or traders the Justice Department is targeting in its criminal probe.

The Justice Department and the banks declined to comment.

Libor is set every day in London for 10 currencies for a range of maturities. In the derivatives market, Libor is used in the pricing of the interest-rate swaps market, where two parties swap floating- and fixed-rate interest payments. Libor typically is used as the basis for the floating rate.

The investigations are examining whether traders at the banks tried to influence whether the rate went up or down. A change in the rate could mean a windfall of tens of millions of dollars if a trader has bet correctly on the direction of Libor, Reuters said.