UnitedHealth Group (UNH) shares plunged 10.86% in pre-market trading on Thursday, following a report that the U.S. Department of Justice is conducting a criminal investigation into the company for possible Medicare fraud. This latest setback compounds recent challenges that have battered the healthcare giant'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.

This news comes just days after UnitedHealth suspended its 2025 financial guidance and announced the abrupt departure of CEO Andrew Witty. On Tuesday, the company cited rising medical costs as the reason for withdrawing its outlook, which triggered an 18% drop in shares to a four-year low.

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 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.