July 9 (Reuters) - Investigators from the U.S. Department of Justice are questioning former employees of UnitedHealth Group as part of a probe into the company's Medicare payment practices, the Wall Street Journal reported on Wednesday, toto20 citing people familiar with the matter.

Shares of the healthcare conglomerate were down 1.7% at $302.47 in early trading.

The WSJ report said former 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 lucrative diagnoses, including testing patients and sending nurses to patients' homes.

UnitedHealth said in a statement it stands "firmly behind" the integrity of it Medicare Advantage business. It also said the WSJ article was "relying on incomplete data, a predetermined narrative and a flawed understanding of how the Medicare Advantage program works."

"After more than a decade of a similar Department of Justice challenge to our Medicare Advantage business, the Special Master concluded there was no evidence to support the claims that we were overpaid or engaged in any wrongdoing."

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

The DOJ 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. UnitedHealth's stock has 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 older 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. (Reporting by Bhanvi Satija and Sriparna Roy in Bengaluru; Editing by Shinjini Ganguli)