UnitedHealth Group shares tumbled Friday on a report that the U.S. Department of Justice has started an investigation into the health care giant's Medicare billing practices.

The market’s reaction was swift and brutal. The stock dropped over 10% in early trading, shaking not just healthcare investors but a significant portion of the US large-cap equity landscape. Why?

The company accuses the newspaper of continuing to report misinformation about Medicare Advantage plans. The Journal published several investigative articles last year about UnitedHealth's billing ...

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UnitedHealth Group stock is sinking Friday on reports the Justice Department is investigating the health insurer. Here's what ...