(Reuters) - UnitedHealth Group ( UNH ) shares fell nearly 6% in premarket trading on Thursday, after the Wall Street Journal reported that the U.S. Department of Justice was investigating the company for potential Medicare fraud.

The latest news adds to the health insurer's litany of woes, including multiple government inquiries into its business, a sudden change in top leadership and a pulled outlook in the face of soaring medical costs.

"UnitedHealth Group ( UNH ) 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.

The company, however, said it had not been notified by the DOJ about the "supposed criminal investigation reported" and that it stood by "the integrity of our Medicare Advantage program".

The healthcare conglomerate's stock sank 18% to a four-year low on Tuesday following the news. If premarket losses hold, UnitedHealth's ( UNH ) market capitalization will halve from $530 billion recorded on April 16, a day before a surprise earnings miss sparked a selloff in its shares.

UnitedHealth ( UNH ) on Monday reinstated Stephen Hemsley as CEO, tasking the longtime leader with steering the healthcare giant through crises including federal probes, soaring costs and a cyber attack.

The health insurance industry has come under intense scrutiny, following a lawsuit earlier this month alleging that three major U.S. health insurers paid hundreds of millions in kickbacks to brokers to funnel patients into their plans.

In February, the WSJ revealed a civil fraud investigation into UnitedHealth's ( UNH ) Medicare practices, while U.S. Senator Chuck Grassley launched an inquiry into the company's billing methods, demanding detailed compliance records, in the same month.

For decades, the company has flourished by leveraging its dominance in insurance, expansion into adjacent areas and growth in the Medicare market, the U.S. government program that covers medical costs for the elderly.

This streak of strong performance came under pressure just months after a senior executive was tragically killed, allegedly by a man enraged over insurer practices.

(Reporting by Joel Jose and Siddarth S in Bengaluru; Editing by Anil D'Silva)