The Dow Jones Industrial Average ended sharply lower Thursday afternoon as the stock market digested a second round of testimony from newly minted Federal Reserve Chairman Jerome Powell and the threat of tariffs on steel and aluminum imports. The Dow DJIA, -2.79% fell 420 points, or about 1.7%, to close at 24,608, with sharp declines in shares of industrial giants United Technologies Corp. UTX and Boeing Co. BA, -2.92% contributing the most to the blue-chip gauge's slump. The Dow had been down by as much as 587 points at its lows of the day. The S&P 500 index SPX, -2.21% finished off 1.3% at 2,677, falling beneath a psychologically significant level at 2,678. Meanwhile, the Nasdaq Composite Index SPX, -2.21% ended down 1.3% at 7,180. Stocks have been volatile but took a leg lower amid reports that President Donald Trump would impose tariffs on steel and aluminum imports. "We're going to build our steel industry back and we're going to build or aluminum industry back," Trump said during a news conference on Thursday. "We're going to be instituting tariffs next week," the president said. Trump's comments, from a White House gathering with leaders in the steel and aluminum industry, raised the specter of protectionist trade policies that unsettled investors--at least in the short term. Meanwhile, earlier Thursday, Powell attempted to moderate comments he made on Tuesday in congressional testimony by emphasizing that the economy wasn't overheating.