Rep. Maxine Waters (D-Calif.) requested the Securities and Exchange Commission investigate a possible leak of the Standard & Poor's downgrade of the U.S. credit rating last week. Stocks tumbled on Aug. 4 amid the first of sizable sell-offs still reverberating through the market over the past few trading days. On Friday morning, Aug. 5, S&P alerted the Treasury Department of its upcoming downgrade of the nation's AAA rating to AA+. When Treasury officials found a $2 trillion error in the numbers, the credit rating agency delayed the action until later that night. "Given the historic nature of S&P’s downgrade, and the tremendous market volatility experienced after that downgrade, I feel it is appropriate for the (SEC) to conduct an investigation into whether S&P selectively disclosed information related to the U.S. government debt downgrade to any financial institutions, and whether any institutions that had that non-public information traded on that information prior to the official announcement," Waters wrote. A Senate Banking Committee aide said they too are looking into the issue, but no investigation had been officially launched. S&P wasn't immediately available for comment. Waters also called for the House Financial Services Committee to hold hearings on the possible consequences of the downgrade. Early signals of the outcome appeared this week. The European debt crisis drove many investors into Treasurys and as a result pushed mortgage rates to new lows. The secondary mortgage market weathered the action as well, despite the resulting downgrade of Fannie Mae and Freddie Mac. Jaret Seiberg, an analyst at MF Global, said the psyche of such a downgrade could hold more dire consequences later on for U.S financial institutions. "The real worry is that economic conditions deteriorate to the point where the banks start suffering large losses again on consumer, mortgage and commercial credit," Seiberg said. "So shocks such as the European debt crisis or U.S. downgrades that spook the markets end up creating turmoil in the real economy, which could then hurt the banks." Write to Jon Prior. Follow him on Twitter @JonAPrior.