(Reuters) – The U.S. Federal Reserve’s regional presidents of banks responded to Senator Elizabeth Warren’s criticism of their ethics guidelines with a joint letter inviting the Fed Board of Governors to review the matter and promising to implement any necessary changes.

The presidents stated that Federal Reserve Chair Jerome Powell had recently directed staff at the Board of Governors to review the ethical rules concerning financial holdings and activities of senior Federal Reserve officials. A copy of this letter was seen by Reuters. “We welcome this important review and will apply any resulting changes to each Federal Reserve Banks’ Code of Conduct.”

This letter was signed by all bank presidents and was signed by James Bullard, President of St. Louis Fed.

Warren had written to banks earlier in September asking for tighter ethics standards, including the ban on senior Fed officials trading individual stocks and prohibiting their ownership.

This was in response to a controversy over Robert Kaplan, Dallas Fed President, and Eric Rosengren (Boston Fed President), who were involved in controversial trading last year when the central bank attempted to rescue the economy. Last month, both said they were leaving their posts despite clearing their trades by Fed ethics officers.

Powell now wants a thorough review of ethical rules regarding financial transactions and financial holdings at central banks. Powell has asked for the Fed’s inspector general to investigate the transactions of certain high-ranking officials.