Some investors, like Anton Bayer, CEO of Up Capital Management in Granite Bay, Calif., believe that financial markets will falter when the Fed and other central banks pump less money into the system. The Fed has artificially propped up the economy, he thinks, which is why investors are nervous about what will happen when the central bank starts buying fewer bonds every month.
"What the markets are seeing is the economic engines are not being primed," Bayer said. "The fear is of the stimulus going away and exposing an economy that is not really chugging along. It's the big risk."
In the U.S. government bond market, the yield on the 10-year Treasury note dropped to 2.14 percent from 2.23 percent late Wednesday.
In Japan, the benchmark Nikkei 225 index slumped 6.4 percent as doubts grew that Prime Minister Shinzo Abe's economic turnaround plan will succeed. The Japanese market is down 20 percent from a high reached May 22, the definition of a bear market.
That decline followed an extraordinary surge from mid-November to late May. The Nikkei soared 80 percent as investors hoped Japan would finally emerge from its two-decade economic slump.
The price of crude oil rose 13 cents to $96.01 a barrel in New York. Gold dropped $13.70 to $1,378.30 an ounce.
Among stocks making big moves:
— Safeway jumped $1.71, or 7 percent, to $24.82 after the company said late Wednesday that it would sell its supermarket business in Canada to food retailer Sobeys for $5.7 billion.
— Gannett soared $6.75 to $26.60 after announcing its deal to buy Belo for $1.5 billion.
— Williams Companies fell 33 cents, or 1 percent, to $33.70. A fire at the company's Louisiana petrochemical plant sent its stock down. The plant makes highly flammable gases, ethylene and propylene.
AP Business Writer Christina Rexrode contributed.