(Reuters) -The U.S. Justice Department has launched an investigation into UnitedHealth's Medicare billing practices in recent months, the Wall Street Journal reported on Friday, citing people familiar with the matter.

Shares of the company fell nearly 10% in premarket trading. Shares of other health insurers also fell before the bell.

The new civil fraud investigation is examining UnitedHealth's practices for recording diagnoses that trigger extra payments to its Medicare Advantage plans, including at physician groups the insurance giant owns, the report added.

DoJ and UnitedHealth did not immediately respond to Reuters requests for comment.

Medicare Advantage plans are offered by private insurers who are paid a set rate by the U.S. government to manage healthcare for older people looking for extra benefits not covered in the regular Medicare coverage. The DoJ has a long-running lawsuit pending against UnitedHealth over Medicare Advantage.

The department had also sued to block UnitedHealth's $3.3 billion acquisition of Amedisys Inc, citing concerns that the deal would reduce competition in the home health services market.

UnitedHealth runs the country's largest health insurer, UnitedHealthcare, as well as pharmacy benefit manager Optum and medical practices.

The company and its peers, who provide government-backed Medicare and Medicaid insurance plans for low-income individuals, have seen an increase in medical costs over the last few quarters.

The company also encountered other challenges last year, including a cyberattack on its tech division Change Healthcare and a backlash over denial of insurance claims after the murder of UnitedHealthcare CEO Brian Thompson.