• UnitedHealth's market cap halved since November peak

• Worst performing Dow component this year

• Not notified about the probe, company says

May 15 (Reuters) - UnitedHealth Group UNH.N shares plunged 11% on Thursday as investors fled the stock after a Wall Street Journal report that the U.S. Department of Justice was carrying out a criminal investigation into the company for potential Medicare fraud.

The reported probe adds to a string of recent setbacks for the healthcare conglomerate that has erased more than half, or over $300 billion, from its market value since November, when shares were trading at record highs.

"The stock is already in the doghouse with investors, and additional uncertainty will only pile on," said James Harlow, senior vice president at Novare Capital Management, which owns shares in UnitedHealth.

The news of the probe follows CEO Andrew Witty's abrupt departure and the withdrawal of its 2025 forecast, which triggered an 18% drop in shares to a four-year low on Tuesday.

"UnitedHealth Group is mired in a crisis seemingly without end. Investors are bracing for another big bout of turbulence given reports of the DOJ investigation," said Susannah Streeter, head of money and markets at Hargreaves Lansdown.

UnitedHealth said it had not been notified of the investigation by the DoJ. The WSJ in February also reported a civil fraud investigation into UnitedHealth's Medicare practices, which the company had at that time said it was not aware of.

The health insurance industry, as well as drug middlemen known as pharmacy benefit managers, has increasingly come under government and public scrutiny.

For decades, the company has flourished by leveraging its dominance in insurance and growth in the Medicare market, the U.S. government program that covers medical costs for the elderly. The focus now turns to its next steps.

The company has tapped former CEO Stephen Hemsley to replace Witty, becoming the latest company to turn to an experienced executive to take the helm, in the hopes of quelling investor concerns amid economic uncertainty.

Despite recent adversities, Oppenheimer analyst Michael Wiederhorn maintained that the company's fundamentals are solid, though he acknowledged that "it may take time to win back the marketplace's confidence."