Washington DC - Insurance giant UnitedHealth Group is reportedly facing a criminal investigation by the Justice Department for possible Medicare fraud.

Authorities are probing the firm's Medicare Advantage program, a scheme where private companies manage the delivery of government health benefits for seniors, the Wall Street Journal reported, citing unnamed sources familiar with the matter.

The report is that latest struggle to hit America's largest health insurer, coming just days after its CEO unexpectedly announced he would be stepping down.

It also follows the high-profile killing last December of Brian Thompson, head of the company's insurance division, in a case that has brought to the surface deep public frustration with the exploitative commercial health care system.

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UnitedHealth said in a statement it had not been notified by the Justice Department of the supposed investigation and criticized the "deeply irresponsible" article.

The Wall Street Journal previously reported in February that UnitedHealth was under investigation over allegations it directed doctors to make patient diagnoses that would trigger high government payments under Medicare Advantage.