Trading in the stock market turned more treacherous in late-day trading as investors reacted to slowing economic growth in China and a massive commodity sell-off -- and as initial word spread as well of the explosions at the end of the Boston Marathon. The Dow Jones industrial average lost 266 points on the day, down 1.8%. It was the Dow's biggest point drop since Nov. 7, in the wake of President Obama's reelection, when the index lost 295 points.