U.S. Attorney Jeanine Pirro had subpoenaed Powell but a court threw out the effort, which Pirro has vowed to appeal. Pirro in recent days referred the investigation to the Fed's inspector general, removing the criminal element and helping clear a political roadblock that had threatened to stall Warsh's confirmation.

Wall Street expects the Federal Reserve to hold interest rates steady at what is possibly Jerome Powell's final FOMC meeting as Fed chair on Wednesday. Many investors are speculating whether Powell will stay on the Fed's Board of Governors after his term at the US central bank concludes on May 15.