By Anora Mahmudova and Barbara Kollmeyer, MarketWatch

NEW YORK (MarketWatch) -- U.S. stock investors turned skittish, sending prices lower on Monday as they fretted about falling commodity prices and concerns about global growth.

Treasurys rose as investors sought safety, while prices of metals dropped sharply.

A decline in economic activity in August, measured by Chicago Federal Reserve as well as a decine in existing-home sales contributed to the dour Wall Street mood.

The S&P 500 (SPX) was off 7 points, or 0.3%, at 2,003.43. The Dow Jones Industrial Average (DJI) slipped 27 points, or 0.2%, to 17,253.02. The Nasdaq Composite (RIXF) fell 25 points, or 0.6%, to 4,566.04.

Data released on Monday, shows U.S. economic activity was lackluster in August. Sales of existing homes unexpectedly declined in August, for the first time in five months, the National Association of Realtors reported Monday. NAR attributed the drop to fewer all-cash sales to investors.

Meanwhile, markets will be poring over comments from a pair of Federal Reserve officials: New York Fed President William Dudley and Minneapolis Fed President Narayana Kocherlakota, Dudley, a voting member of the Fed policy committee is set to speak at Bloomberg Markets Most Influential Summit at 10 a.m. Eastern, while another voting member Kocherlakota will give a speech on the objectives of monetary policy to the Economic Club of Marquette County at 7:30 p.m. Eastern.

Clues about the Fed's timing intention on hiking rates and news from China are what traders will be looking out for, said Joao Monteiro, analyst at Valutrades.

China worries festering: Hong Kong's Hang Seng Index fell to two-month lows on Monday as traders worried ahead of the HSBC estimate for China's September purchasing managers index, due after the close of U.S. markets.

Any number under 50 -- indicating contraction -- could mean more losses for Asia markets, barring meaningful stimulus measures. At a G-20 meeting over the weekend, China's Finance Minister Lou Jiwei said the country is facing downward pressure, but won't "make major policy adjustments" due to changes in any individual economic indicator.

"We're not going to see this wall of money thrown at the Chinese slowdown," Stuart Beavis, head of institutional equity derivatives at Vantage Capital Markets in Hong Kong, told Bloomberg News.

Stocks to watch: Alibaba shares fell 2.3% on the second day of trading. With the e-commerce company's underwriters set to exercise an option to sell additional shares, the IPO is now officially the world's largest, Dow Jones Newswires reported Sunday.

Shares in Apple, Inc.(AAPL) rose 0.4%, after the company said iPhone 6 sales topped 10 million during the first weekend, a new Apple record.

AutoZone Inc. (AZO) reported fiscal-fourth quarter earnings that topped estimates, but sales were slightly below forecasts. Shares were off 2.4%.

Dresser-Rand Group Inc. (DRC) gained 2.7% after German engineering company Siemens AG announced a deal to buy the U.S. oil-equipment maker for $7.6 billion.

Shares of Sigma-Aldrich Corp. soared 35% in premarket trade after Merck & Co. Inc. (MRK) said Monday it will pay $140 per share for the life-science and technology company, a price that values the company at around $17 billion. (Read more about the day's notable movers here: http://www.marketwatch.com/story/alibaba-autozone-in-spotlight-2014-09-21.)

Other markets: Silver (SIZ4) prices fell to four-year lows on Monday on dollar strength. Gold (GCZ4) prices edged lower as well, falling $2.8 to $1,213.8 an ounce. The dollar strengthened in early New York trade.

In London, shares of U.K. grocer Tesco PLC fell more than 8% after a profit the company announced that it launched an internal probe of its accounting practices, suspended top executives and was forced to restate its first-half 2014 results.

China worries also took a toll on mining shares, cutting into the FTSE 100 index , with other European indexes slumping.

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