NEW YORK — Fear has taken over on Wall Street. The Dow Jones industrials fell 634.76 points, the first trading day since Standard & Poor’s downgraded American debt. . It was the sixth-worst point decline for the Dow in the last 112 years and the worst drop since December 2008. Every stock in the Standard & Poor’s 500 index declined Monday.

But the S&P downgrade wasn’t the only catalyst Monday. Investors worried about the slowing U.S. economy, escalating debt problems threatening Europe and the prospect that fear in the markets would reinforce itself, as it did during the financial crisis in the fall of 2008.