The Dow Jones industrial average fell 167.16 points or 1.01 percent, to end at 16,446.81. The S&P 500 lost 17.68 points or 0.94 percent, to 1,870.85. The Nasdaq Composite dropped 31.33 points or 0.76 percent, to 4,069.29.
 

Thursday marked the second day of losses for the three major U.S. stock indexes, with volume picking up on S&P 500 e-mini futures, underscoring some views that the selling may continue. At 2.2 million, e-mini futures volume was well above the 1.5 million daily average for the past year.

The Russell 2000 index of small-cap stocks lost 0.7 percent, well off its session low where it hit correction territory, down more than 10 percent from its early March record close of 1,208.65. It ended down 9.3 percent from that high.

With the Dow and the S&P 500 near record highs, the Russell 2000's move raised concern about the strength of the broader market.

The Russell 2000's move "either is a leading market indicator or it could be a one-off ... and that's yet to be determined," said Adam Sarhan, chief executive of Sarhan Capital in New York.

Stocks are "starting to form a little bit of a ceiling," said Uri Landesman, president of Platinum Partners in New York. "The small caps have really been selling off, and I think you're continuing to see a rotation out of anything the market considers riskier."

Wal-Mart Stores shares fell 2.4 percent to $76.83, weighing on both the Dow and the S&P 500. The world's largest retailer forecast second-quarter profit below analysts' estimates. Wal-Mart reported its smallest growth in quarterly sales in nearly five years and a drop in first-quarter profit, saying that severe winter weather made it difficult for customers to visit its stores.

The CBOE volatility index rose 8.2 percent. The spread between the spot VIX and the 3-month VIX at one point tightened to its smallest since April 15.

The S&P 500 briefly traded below technical support, its 50-day moving average, but ended above that level.

Adding to bearish views, billionaire hedge fund manager David Tepper sounded cautious late Wednesday, saying he was "nervous" about the stock market but that this was not the time to sell.

Economic data was mixed, though U.S. industrial output fell at its fastest rate in more than 1-1/2 years in April as factory production slumped, tempering hopes for a big jump in economic growth after a winter slowdown. Initial claims for U.S. jobless benefits hit a seven-year low last week, while consumer prices recorded their biggest increase in 10 months in April, pointing to a firming economy.

Among the day's gainers, Cisco Systems jumped 6 percent to $24.18, a day after the network equipment maker posted a shallower-than-expected drop in quarterly revenue.