Technology companies led a broad sell-off in stocks Wednesday, knocking more than 600 points off the Dow Jones Industrial Average and handing the market its worst day in nearly three months. The S&P 500 fell 2.6%, its biggest single-day drop since it lost 3.5% on October 28. It had set a record high just two days earlier. The Dow and tech-heavy Nasdaq composite also fell more than 2%. The sell-off left the S&P 500 and Dow in the red for the year.