The U.S. stock market experienced its worst day of 2025 on February 21, with the Dow Jones Industrial Average dropping 748.63 points, or 1.69%, to close at 43,428.02. The S&P 500 also fell by 1.71% to end at 6,013.13. This significant decline was largely driven by a sharp drop in UnitedHealth Group's stock, which fell more than 7% after reports emerged that the Department of Justice (DOJ) is investigating the company's Medicare billing practices. The DOJ's civil fraud investigation focuses on how UnitedHealth records diagnoses that trigger extra payments for its Medicare Advantage plans. The Wall Street Journal reported that the investigation is looking into whether the company inflated diagnoses to receive higher federal payments. UnitedHealth responded by stating it was unaware of any new DOJ probe and criticized the report as containing misinformation. The market's reaction was not limited to UnitedHealth; other Medicare Advantage insurers like Humana also saw their stocks decline. The broader market was affected by concerns over economic growth, with nearly $927 billion wiped out from the U.S. stock market on that day.