The Dow Jones Industrial Average on Thursday was down over 400 points and the S&P 500 index was trading beneath an intermediate-term trend line, putting both bechmarks on track to post their worst September declines since 2011, according to Dow Jones Market Data. At last check, the Dow was down 440 points, or 1.3%, to reach around 33,946, down 4% on the month, and on the verge of the ugliest September performance since the index tumbled 6% in September of 2011. The S&P 500 index was trading 0.7% lower Thursday to reach around 4,326, with that downward pressure pushing the broad-market benchmark below its 100-day moving average at 4,344.58. Market technicians view moving averages as dividing lines between bullish and bearish momentum in an asset. The S&P 500’s slump on Thursday was contributing to a 4.3% drop for the month and setting the stage for its worst September since a 7.2% drop in 2011.