Some financial commentators said the worst was over after the S&P 500 index rebounded 6.4 percent from Aug. 25 to Aug. 28, following a plunge of 11 percent from Aug. 17-25.

But now that the index has slid nearly 6 percent so far this week, worry is creeping back into the market.

“We were in a bull market in complacency,” star hedge fund manager Doug Kass, president of Seabreeze Partners Management, told The New York Times.

“I think that the spike in volatility that we’ve seen in the last 10 days is the precursor to further spikes.”

The CBOE Volatility Index (VIX) has soared 61 percent during that period to 30.91.

“The fundamentals around the world are still wobbly, and valuations are still elevated,” Kass said. As for valuations, the trailing price-earnings ratio for the S&P 500 index totaled 20.69 Friday, up from 19.19 a year ago, according to Birinyi Associates.

Meanwhile, weak data continue to pour out of the economies of the eurozone, Japan and China, not to mention other emerging markets.

Doug Ramsey, chief investment officer at Leuthold Group, doesn't think the trouble is over for stocks either. The problem is a shortage of cash that investors can use to buy equities after the Federal Reserve ended its quantitative easing program in late 2014, he told The Wall Street Journal.

The Fed was purchasing $85 billion of bonds a month.

In addition, Ramsey calculates the S&P 500's price-earnings ratio, using five years of earnings, at 20, up from a long-term median of 17.

The weakening of transportation, utility and industrial shares also has served as a warning sign for the market, Ramsey says. This looks like “a garden-variety, cyclical bear market,” meaning a decline of 20 to 25 percent, he said.

A 25 percent drop from the S&P 500's May 20 record high of 2,134.72 would put the index at 1,601. Tuesday, it fell 58 points, or 3 percent, to 1,913.

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