U.S. prosecutors have expanded their probe of currency-market manipulation by some of the world's largest banks to include the Russian ruble and Brazilian real, according to two people familiar with the matter.

The Justice Department is using cooperation agreements it reached with banks in May to gather information and interview traders about suspected market rigging, said the people, who asked not to be named because the investigation is confidential. Trading of the Argentine peso has also attracted the attention of U.S. prosecutors, one of the people said.

The escalating investigation is scrutinizing trading practices at banks that didn't previously settle currency-rigging claims. Prosecutors are examining a handful of institutions, including Deutsche Bank AG, the two people said. The bank has already disclosed it's under a half-dozen criminal and regulatory investigations, including for possible currency manipulation.