The U.S. Justice Department’s criminal healthcare-fraud unit has opened an investigation into UnitedHealth Group’s Medicare Advantage billing practices, according to people familiar with the matter. Prosecutors are examining whether the company used doctors and nurses to collect diagnoses that boosted risk-adjusted payments from the federal program. In recent weeks investigators from the Justice Department, the Federal Bureau of Investigation and the Department of Health and Human Services’ Office of Inspector General have questioned former UnitedHealth employees about how the insurer gathered and reported patient data, the Wall Street Journal reported. UnitedHealth said a court-appointed Special Master previously concluded there was no evidence the company was overpaid or engaged in wrongdoing and cited Centers for Medicare & Medicaid Services audits showing its coding accuracy among the industry’s best. The Journal’s report sent UnitedHealth shares down about 1% in pre-market trading on Wednesday.