KEEPING SCORE: The Standard & Poor's 500 index fell 18 points, or 1 percent, to 1,919 as of 10:15 a.m. Eastern time. The Dow Jones industrial average dropped 188 points, or 1.2 percent, to 16,083. The Nasdaq composite fell 49 points, or 1.1 percent, to 4,702.
FED WATCH: Many investors are awaiting a speech on inflation and monetary policy later Thursday by U.S. Federal Reserve chair Janet Yellen. Markets are looking for clues on the timing of the Fed's first interest-rate hike in nearly a decade after it held off raising rates earlier this month. The Fed has rate-setting meetings in October and December.
CHALLENGING TIMES: Caterpillar fell $4.80, or 6.7 percent, to $65.41 after cutting its 2015 revenue forecast by $1 billion to about $48 billion, and saying sales would fall a further 5 percent next year. The company also said it may cut as many as 10,000 jobs between now and 2018.
EUROPE'S DAY: Germany's DAX fell 2.3 percent, Britain's FTSE 100 dropped 1.1 percent and France's CAC 40 1.1 percent.
AUTO SHARES: Automakers fell in the wake of Volkswagen's emissions scandal. After early gains, Volkswagen shares were down 1 percent. Fellow German carmaker BMW fell 5.2 percent after a report said one of its models had failed a test in Europe. France's Renault was down 2.8 percent.
ENERGY: Benchmark U.S. crude was up 29 cents at $44.75 a barrel on the New York Mercantile Exchange.
BONDS AND CURRENCIES: Bond prices rose, pushing the yield on the 10-year note down to 2.09 percent from 2.15 percent a day earlier. The dollar slipped to 119.57 yen. The euro ticked higher to $1.1241.