(Reuters) -The U.S. Department of Justice (DOJ) is carrying out a criminal investigation into UnitedHealth Group for possible Medicare fraud, the Wall Street Journal reported on Wednesday.

The company's stock slipped 8% in after-hours trade following the report, which follows a series of problems at the insurer.

On Tuesday, UnitedHealth Group's CEO, Andrew Witty, stepped down unexpectedly, and the company simultaneously suspended its 2025 financial forecast due to rising medical costs.

The announcement sent shares plunging nearly 18% to a four-year low.

While the exact nature of the criminal allegations is unclear, the Journal said, citing people familiar with the matter, the probe by the healthcare-fraud unit of the DOJ's criminal division has been active since at least last summer.

In February, the Journal reported a civil fraud investigation into UnitedHealth's Medicare practices. The company had then said that it was unaware of any new probe.

In the same month, U.S. Senator Chuck Grassley launched an inquiry into UnitedHealth's Medicare billing practices, requesting detailed records of their compliance programme and other related documents.

A DOJ spokesperson declined to comment to the Journal about the fresh criminal probe. The UnitedHealth Group and the DOJ did not immediately respond to Reuters requests for comments.

The new investigation follows a broader scrutiny into the Medicare Advantage program.

The DOJ, earlier this month, had filed a lawsuit accusing three of the largest U.S. health insurers of paying hundreds of millions of dollars in kickbacks to brokers in exchange for steering patients into the insurers' Medicare Advantage plans.

Nearly half of the 65 million people covered by Medicare, the U.S. programme for people aged 65 and older or with disabilities, are enrolled in Medicare Advantage plans run by private insurers.

The insurers are paid a set rate for each patient, but can be paid more if patients have multiple health conditions.

Standard Medicare coverage is managed by the government.

(Reporting by Bipasha Dey and Kanjyik Ghosh in Bengaluru; Editing by Sumana Nandy and Arun Koyyur)