(Reuters) -Investigators from the U.S. Department of Justice are questioning former employees of UnitedHealth Group ( UNH ) as part of a probe into the company's Medicare payment practices, the Wall Street Journal reported on Wednesday, citing people familiar with the matter.

Shares of the healthcare conglomerate were down nearly 2% in premarket trading.

The report added that the employees were questioned by prosecutors working for the healthcare-fraud unit in recent weeks about the company's efforts to record specific diagnoses that generate higher payments and document certain lucrative diagnoses, including testing patients and sending nurses to patients' homes.

The Federal Bureau of Investigation and the Department of Health and Human Services' Office of Inspector General also participated in some of the interviews, the report added.

UnitedHealth and the Department of Justice did not immediately respond to a Reuters request for comment.

Once considered a reliable bet, the healthcare behemoth has lost its appeal following a series of management missteps. Shares of UnitedHealth have fallen nearly 40% so far this year.

The Wall Street Journal reported in May that the healthcare-fraud unit was overseeing an investigation into the company's Medicare business, the U.S. government program that covers medical costs for individuals aged 65 or more and those with disabilities.

The report came a day after UnitedHealth suspended its annual forecast due to surging medical costs and said that former CEO Stephen Hemsley would replace Andrew Witty.

Hemsley, who had led the company for more than a decade until 2017, has his task cut out for him. The company is facing a series of challenges, including rising medical costs, regulatory scrutiny of its drug pricing negotiations and reported probes on its billing practices.

He has vowed to restore shareholders' trust after a surprising earnings shortfall earlier this year, saying the company was evaluating medical cost trends and how they could affect its future performance.

(Reporting by Bhanvi Satija and Sriparna Roy in Bengaluru; Editing by Shinjini Ganguli)