The Wall Street Journal on Wednesday cited people familiar with the matter as saying that the probe has been ongoing since last year.

Shares of UnitedHealth Group plunged more than 13% 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 has dropped nearly 50% amid headwinds, but strong fundamentals, margin stability, and low valuation signal upside. See why UNH stock is a buy.

Shares of UnitedHealth Group ( UNH -5.75%) plummeted again on Wednesday. The company's stock is down 5.8% as of market close, while the S&P 500 ( ^GSPC -1.61%) and the Nasdaq Composite ( ^IXIC -1.41%) lost 1.6% and 1.4%, respectively.

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.

Health insurance giant UnitedHealth Group saw its stock price sink again on Thursday on rumors that it is under investigation by the DOJ.

Few stocks have been as resilient as UnitedHealth Group ( UNH 8.14%). The company operates the largest health insurance business in the U.S. It runs the second biggest pharmacy benefits manager (PBM). And UnitedHealth Group has a fast-growing healthcare technology and services unit with Optum.

UnitedHealth shares slid in recent trading Thursday after tumbling Wednesday amid worries about potential cuts to Medicare, and a report accusing the insurer of paying secret bonuses to nursing homes.

UnitedHealth Group (UNH) stock defended by analysts as the company takes heat following a report on a DoJ criminal probe on its Medicare business. Read more here.