US healthcare giant UnitedHealth Group is under criminal investigation by the Department of Justice (DOJ) for possible Medicare fraud, the Wall Street Journal reported on Wednesday, adding that the investigation specifically concerns the company's practices under its Medicare Advantage program.

According to the newspaper, the DOJ's healthcare fraud unit is overseeing the investigation, which has been ongoing since at least last summer. Specific details of the potential charges have not been disclosed.

In a statement, UnitedHealth said it had not been informed of any criminal investigation and defended the integrity of its Medicare Advantage program. The news nevertheless caused UnitedHealth's stock to fall 8% in after-hours trading on Wednesday.

This revelation comes at a particularly difficult time for the group. On Tuesday, its CEO Andrew Witty unexpectedly stepped down, and the company suspended its financial forecasts for 2025 due to soaring medical costs, causing its shares to plummet nearly 18% to their lowest level in four years. Stephen Hemsley, the group's former CEO until 2017, has been called back to lead the company in an attempt to stabilize the situation.

The case is part of a broader context of increased scrutiny of the Medicare Advantage program, an alternative to standard Medicare managed by private insurers. The program now covers nearly half of the 65 million Medicare beneficiaries in the United States. Insurers are paid a flat fee per patient, but can receive higher payments if the insured have multiple conditions. This incentive mechanism is under intense scrutiny by the authorities.

In February, the Wall Street Journal revealed a civil fraud investigation into UnitedHealth's billing practices in Medicare. The company said at the time that it was not aware of any new proceedings. That same month, Republican Senator Chuck Grassley launched his own congressional investigation, demanding detailed documents from UnitedHealth on its compliance procedures.

Just last week, UnitedHealth admitted in a regulatory filing that it was "involved or currently involved in various government investigations, audits and reviews," without specifying their nature.

Meanwhile, the DOJ filed a lawsuit this month against three major US health insurers, accusing them of paying hundreds of millions of dollars in kickbacks to brokers to steer patients toward their Medicare Advantage contracts.

Since the beginning of the year, UnitedHealth's share price has lost around 40% of its value, weighed down by the investigations, the governance crisis and the media fallout from last December's murder of Brian Thompson, then head of the group's insurance division. The murder sparked a wave of criticism on social media targeting the industry's practices, further exacerbating the difficulties faced by the health insurance giant.