American stocks and bond yields fell sharply as concerns mounted that the economy might be in for a slowdown.

The Dow Jones Industrial Average fell 2.6% back below 10000 on slumping U.S. consumer confidence, the downward revision of a Chinese economic indicator and worries about the health of Europe's banks. Yields on 10-year Treasury notes fell below 3% to their lowest level since April 2009, a sign investors were seeking out safe investments.

The...