The Dow Jones industrial average closed down 725.81 points, or nearly 2.1 percent, to 33,962.04 for its worst day of 2021. The S&P 500 index skidded 68.67 points, nearly 1.6 percent, to settle at 4,258.49. The tech-heavy Nasdaq composite index shed 152.25 points, or nearly 1.1 percent, to close at 14,274.98.