The company has said in regulatory filings that investigations, audits and reviews by a dozen different government agencies are currently underway.

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 is investigating UnitedHealth Group for possible criminal Medicare fraud, people familiar with the matter said. The healthcare-fraud unit of the Justice Department’s criminal division is overseeing the investigation,