The federal government sued UnitedHealth Group on Tuesday alleging the Minnetonka-based health care company wrongly received from Medicare at least $1 billion in risk adjustment" payments based on inaccurate data submissions. The federal government's civil fraud action comes in a whistleblower case first brought by a former UnitedHealth Group employee. Earlier this year, the federal government disclosed it had ongoing investigations about risk adjustment practices at four other carriers including Aetna and a division of Cigna. In Medicare Advantage plans, the government pays health insurers a per-member per-month payment for enrollees. The government says the fees can be increased when health plans submit information about an enrollee's health that justifies a higher "risk score" for the patient. The federal lawsuit filed Tuesday highlighted UnitedHealth's program to review charts, calling it a "one-sided revenue-generating program." The insurer collected "millions of medical records" and employed chart reviewers "in order to mine for diagnoses that the providers themselves did not report to United for their patients," the lawsuit states. "United used the results of the chart reviews to only increase government payments ... while in bad faith systematically ignoring other information from the chart reviews which would have led to decreased payments."