Wall Street ended slightly lower, yesterday, on concerns that the U.S. Federal Reserve could raise interest rates sooner than later. San Francisco Fed President John Williams and his St. Louis counterpart, James Bullard, both struck hawkish tones, yesterday. Last week, investors were surprised at central bank minutes that opened the door to a rate hike as soon as June. Investors will listen for fresh clues to the Fed's intentions when Chair Janet Yellen speaks on Friday. So the Dow Jones industrial average fell 8 points, to 17,493. The Standard & Poor's 500 index lost 4 points, or 0.2 percent, to 2,048. And the Nasdaq composite shed 4 points, or 0.1 percent, to 4,766. Tightening borrowing costs would help choke inflation, but also hamper economic expansion and reduce liquidity in stock markets, which could impede stock gains.