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,

The Justice Department’s healthcare-fraud unit is leading the probe, the Journal reported. UnitedHealth Group’s stock tanked around 7% following the Journal’s report, bringing shares to their lowest point in almost five years.