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 timing of the reported investigation follows closely on the heels of a recently filed DOJ lawsuit that alleges a large-scale kickback scheme involving health insurers and brokers aimed at boosting Medicare Advantage enrollments.