Stocks turned sharply lower in afternoon trading on Wall Street as talk of steep tariffs on steel and aluminium spooked investors.
The Dow Jones industrial average dropped as much as 500 points on Thursday.
Industrial companies that would take a hit from higher steel and aluminium prices fell sharply.
Heavy equipment maker Caterpillar fell 2% and aerospace giant Boeing gave back 4%.
Big exporters like Apple and drugmaker Pfizer, which would suffer if trade tensions picked up, also fell.
The Standard & Poor's 500 index dropped 41 points, or 1.5%, to 2,671. The benchmark index is coming off its worst month in two years.
The Dow was down 470 points, or 19%, at 24,554. The Nasdaq composite fell 127 points, or 1.8%, to 7,142.