It was The Day After on Wall Street -- the uneasy end to a hair-raising week that, for a few terrifying moments, left the financial world stunned. After Thursday's near panic, shaken traders returned to their desks Friday morning to grapple with the aftermath of one of the wildest days in Wall Street history. But even as the exact cause of the 1,000-point plunge on Thursday remained unclear, many traders seemed to put the episode behind them -- if only to confront some terrible 'ifs' now weighing on the world's market. Can Greece pay its debts? What about Portugal or Spain? Can a new prime minister reduce Britain's gaping budget deficit? Will the crisis rocking Europe spread? And what about the American economy? Investors hate uncertainty, but uncertainty -- lots of it -- is what they are facing. Add to that Thursday's white-knuckled ride and the result, for some, is a case of sky-high anxiety that could cause more wild market swings. 'Clearly there is still an aftershock from yesterday,' said Uri Landesman,the president of Platinum Partners. 'People are scared of the magnitude of the panic that caused, and they are realizing, 'Hey, people are really nervous.'' The stock market rounded out the week with what was, next to Thursday, a relatively modest decline. The Dow Jones industrial average fell 139.89 points, or 1.33 percent, to 10,380.43, while the Standard & Poor's 500-stock index declined 17.27 points, or 1.53 percent, to 1,110.88.The Nasdaq composite dropped 54 points, or 2.33 percent, to 2,265.64. European indexes closed sharply lower. It was the culmination of a week in which the Dow lost about 5.7 percent; the S.&P. 500 fell 6.4 percent, and the Nasdaq 7.5 percent. Investors remained on edge on Friday, and the stock market continued to be buffeted by bouts of volatility. The Dow sank briefly after the opening bell but within minutes began to rally and for a moment pulled into the black before tumbling anew. It slipped into positive territory twice more before lunch time, and then ground lower through the afternoon. The unease was reflected in the VIX -- a volatility measure sometimes referred to as the 'fear index.' On Friday the VIX reached its highest level since March 2009, when the stock market reached its low point. In such an atmosphere, better-than-expected jobs data that would have normally moved markets were brushed aside, even though the 290,000 jobs added to the economy in April exceeded expectations. Or some adapted their investment strategy to take into account the changed mood. Phil Orlando, chief equity market strategist at Federated Investors, said he and others in his firm expected that after Thursday's plunge the market would not be as receptive to the better jobs figures. So on Friday, after the data came out, they organized a hurried conference call. One colleague came in from a day off. Another called in from home. 'We had sort of an emergency meeting,' said Orlando. 'Ordinarily we might have bought into a jobs report like that instead of selling into it. We were going to have to conduct ourselves differently than we ordinarily might have because of this change in psychology.' The lingering queasiness from Thursday's fall made it even harder for investors to overcome their concerns over the financial situation in Europe, let alone react to the favorable jobs data. 'The elements you have seen over the last few days have injected fear back into the equation,' said Doug Roberts,the chief investment strategist for Channel Capital Research. 'People are looking for an excuse to go to the doors.' 'This is what you saw in 2008,' said Roberts. 'As the great Yogi Berra once said, 'It is d?j? vu all over again.'' Gold rose sharply for a second day as investors sought refuge from the volatile stock market. Oil fell $2, $75.11 a barrel. 'People are almost looking at the stock market as a casino, and it turns people off a bit,' William J. Schultz,chief investment officer of McQueen, Ball & Associates, said. 'They would rather stay on the sidelines or flow into bond funds.' 'I sort of feel like the market is really overhung by this,' said Tom di Galoma,of Guggenheim Capital. 'And it is going to take some time for the market to gain some upward direction again.' Wall Street had negotiated some of the turmoil caused by the European debt crisis in the last few weeks. It has managed to close higher on some days, largely springing off promising domestic economic figures. On Friday, uncertainty in the European debt saga eased somewhat when the German Parliament voted to play its part in the Greek aid package. But the exact answer as to why the market reeled Thursday remained elusive. President Barack Obama, in remarks at the White House, said the authorities were looking into it. But 24 hours later, Wall Street had yet to receive an official answer as to what caused the stock market's biggest intraday drop on record. The lack of answers, some said, was bound to undermine investors' confidence . 'Here is the problem,' said Art Hogan, the New York-based chief market analyst at Jefferies & Company. 'We don't have the clear explanation for how it happened.' Hogan said traders, analysts and sales people arrived at Jefferies on Friday morning and immediately began swapping stories about Thursday's plunge. As the day wore on, he and others tried to reassure anxious clients. Hogan said: 'We need clarity on what caused it. We need clarity on what happened in the U.K ., and we need clarity on the European debt.' Stephen Wood, chief market strategist for Russell Investments, said he was on the New York Stock Exchange floor on Thursday when the drop occurred. On Friday, he said the market was still grasping for an explanation as strategists looked to 'iron out the short-term volatility' and put it behind them. 'Two weeks ago we had a raging bull-market mentality,' Hogan said. Now, on top of Greece, comes a startling market plunge, possibly one caused in part by a glitch or trader error, he said. 'It was a volatile, fragile environment going in.' 