The federal government is investigating whether four health insurers have submitted false claims to Medicare, much as Minnetonka-based UnitedHealth Group was accused of doing in a whistleblower case unsealed last month.

In February, prosecutors joined a lawsuit alleging that UnitedHealth Group and health plans that hired one of the company's subsidiaries received hundreds of millions, if not billions, in overpayments due to false claims and statements relating to risk adjustment payments in Medicare.

In a court filing this week, prosecutors disclosed ongoing investigations involving four other health insurers — Aetna, Health Net, Humana and a division of Cigna.

The companies either couldn't be reached for comment, or didn't provide any. Last month, UnitedHealth Group denied the allegations.

"Until those investigations are completed, the United States cannot reach a decision about the liability of these other defendants under the False Claims Act … with respect to the truthfulness of their claims to the Medicare program for risk adjustment payments, the truthfulness of their risk adjustment attestations ... or their possible improper avoidance of returning overpayments," prosecutors said in the filing.

In the whistleblower case, UnitedHealth Group and other insurers were accused of submitting false information about patient conditions to boost payment rates from Medicare.

Medicare health plans make ­payments to doctors and hospitals for enrollees' health care needs, and are paid by the government, in turn, for managing the care. Payments to health plans are adjusted upward when insurers happen to cover patients with more costly health problems, but the lawsuit alleged that insurers were wrongly boosting the risk adjustment claims.

Benjamin Poehling, the former director of finance at UnitedHealth who brought the lawsuit, alleged that insurers submitted forms for diagnoses that health plan members didn't have, or for which members weren't treated in the relevant year. Insurers also claimed that enrollees were treated for more serious conditions than they actually had, according to the lawsuit.

Prosecutors declined to comment Friday. Attorneys for Poehling could not be reached.

Aetna, Health Net and the Cigna division were among the 15 defendants in the whistleblower's original case, but prosecutors last month said they would join the lawsuit only against UnitedHealth Group and a Texas company called WellMed, which UnitedHealth acquired about five years ago.

Prosecutors this week submitted a corrected filing, saying they hadn't yet reached a decision on whether to intervene in the whistleblower's case against the other insurers due to the ongoing investigations.

In whistleblower cases, a "relator" brings a lawsuit on behalf of the federal government, which makes a decision on whether to join the case. False claim cases are filed to recover funds for the government, with relators receiving a portion of any recoveries.

Twitter: @chrissnowbeck