The foreign-exchange trading business was in upheaval across Wall Street as senior executives resigned and others were fired amid an expanding probe of possible manipulation.

Benjamin Lawsky, superintendent of New York's Department of Financial Services, asked more than a dozen firms, including Deutsche Bank AG, Goldman Sachs Group Inc., and Citigroup Inc., for documents on their currency-trading practices, said a person with knowledge of the matter. Deutsche Bank, the top foreign-exchange trader, fired four dealers after an internal probe, people with knowledge of the move said. Goldman Sachs lost two partners, while Citigroup said its foreign-exchange chief will leave in March.

Lawsky's probe is at least the 12th announced by authorities in Europe, the U.S., and Asia since Bloomberg News reported that traders at the world's largest banks colluded to manipulate the benchmark WM/Reuters rates. Even staff who aren't being probed are reassessing career plans as the scandal forces firms to change fundamental practices as revenue falls.