Federal Reserve Chairman Jerome Powell says the central bank’s rules about what type of assets Fed officials can invest in need updating amid an ethics controversy involving regional presidents.

Speaking at a news conference following a two-day meeting of the central bank’s Federal Open Market, Powell was asked about Robert Kaplan, the president of the Federal Reserve Bank of Dallas, and Federal Reserve Bank of Boston President Eric Rosengren. Filings recently revealed that the two made trades last year of securities that could be influenced by Fed policy.