The Justice Department’s years-long court battle to force UnitedHealth Group to return billions of dollars in alleged Medicare Advantage overpayments hit a major setback Monday when a special master ruled the government had failed to prove its case.

Sen. Chuck Grassley (R-Iowa) has given UnitedHealth Group two weeks to explain its method for documenting Medicare Advantage enrollees' diagnoses.

By Brendan Pierson (Reuters) -A court-appointed special master has found that U.S. authorities do not have evidence to support their billion-dollar fraud case accusing UnitedHealth Group of keeping overpayments from the government for patients on its Medicare insurance plans.

The case began with a whistleblower who said UnitedHealth bilked billions of dollars through the Medicare Advantage program.

The Justice Department lacks evidence to support claims the company withheld Medicare Advantage funds, a legal expert overseeing the case said.