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,

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.

Shares of UnitedHealth Group tumbled 16% in intraday trading Thursday, leading S&P 500 decliners, on a report that the U.S. Department of Justice is investigating the embattled health insurance provider for possible criminal Medicare fraud.