The Dow Jones Transportation Average DJT, -2.77% sank 283 points, or 2.7%, in afternoon trade, to push the index below a key technical level that could mark the start of a new longer-term downtrend. The key level is the Feb. 9 closing low of 10,136.61. A close below that level, coupled with the Dow Jones Industrial Average's DJIA, -2.85% close below its February low on March 23, and lower highs reached by both indexes last month, would trigger a sell signal from the century-old Dow Theory of market analysis, suggesting the previous longer-term uptrend was over, and a new downtrend had begun. Meanwhile, the Dow industrials plunged 670 points, or 2.8%, toward a 4 1/2-month low.