Shares of UnitedHealth Group plunged more than 18% following a report that the Department of Justice is carrying out a criminal investigation into the health-care giant over possible Medicare fraud. The DOJ is reportedly focusing on the company's Medicare Advantage business practices,

Key Takeaways Shares of health insurance providers tumbled on a report UnitedHealth Group is being investigated by U.S. officials over possible fraud.The Wall Street Journal said the Department of Justice was looking into the company's Medicare Advantage business.

UnitedHealth Group, one of America’s largest health insurers, is facing a storm of challenges—from a criminal investigation to a leadership shake-up—that have rattled markets and erased billions in shareholder value.