NEW YORK) Stocks accelerated their selloff in heavy volume Monday as investors fled from risky assets following S&P's downgrade of U.S.'s credit rating last week in addition to ongoing uncertainty over the euro zone crisis. The Dow Jones Industrial Average dropped sharply to break below the psychologically-significant 11,000 mark, led by Bank of America and Alcoa, after logging its steepest weekly decline since Mar. 2009 last week. The S&P 500 and the tech-heavy Nasdaq were down almost 5 percent. August is already on track to be the worst month for both indexes since Oct. 2008.