The Dow Jones Industrial Average dropped by 300 points to end below the 6800 mark for the first time in nearly 12 years, as a broad-based selloff seized the markets, sending shares lower in every sector. The S&P 500 briefly dropped below 700 for the first time since October 1996 before ending just at that level amid across-the-board declines, including drops of more than 6% in basic materials, energy, financial and industrial sectors. The Nasdaq Composite Index fell 4%.