UnitedHealth Group (UNH) shares tumbled 6.24% in pre-market trading on Thursday, following reports that the healthcare giant is under criminal investigation by the U.S. Department of Justice (DOJ) for possible Medicare fraud. This latest setback comes on the heels of a series of challenges that have battered the company's stock in recent weeks.

According to The Wall Street Journal, the DOJ's healthcare-fraud unit is overseeing the investigation, which has been active since at least last summer. While the exact nature of the potential criminal allegations remains unclear, sources familiar with the matter indicate that the federal probe is focusing on UnitedHealth's Medicare Advantage business practices. The news compounds the company's recent troubles, which include the abrupt departure of CEO Andrew Witty and the suspension of its 2025 financial guidance due to rising medical costs.

UnitedHealth has stated that it has not been notified by the DOJ of any criminal investigation and maintains that it stands by the integrity of its Medicare Advantage program. However, investors appear to be increasingly wary of the mounting challenges facing the company. The pre-market plunge follows a steep decline of nearly 18% on Tuesday after the announcement of the CEO change and guidance suspension. With the stock having lost almost 50% of its value over the past month, market participants will be closely watching for any further developments in this ongoing saga.