UnitedHealth case involving its Medicare Advantage program unlikely to resolve quickly.

The federal government has increased scrutiny of Medicare fraud in recent years, but no investigation appears bigger than its probe of UnitedHealth Group.

The inquiry is the biggest because UnitedHealth is the nation’s largest provider of Medicare Advantage benefits, a privatized form of Medicare. And United’s Medicare business is the target of not only a civil inquiry, but an uncommon criminal investigation, too.

“The Department of Justice’s actions against [Medicare Advantage insurers] have been civil, focused on recovering money,” said David Lipschutz, co-director of the Connecticut-based Center for Medicare Advocacy. “I don’t recall any criminal cases.”

“The enhanced [civil and criminal] scrutiny of UnitedHealth raises the profile of this investigation,” said Eagan Kemp, a health care policy advocate for Public Citizen, a nonprofit public interest group in Washington, D.C.

The taxpayer-funded Medicare Advantage program — administered by private insurers, including UnitedHealthcare — has grown in popularity over the past 15 years and now draws more members than traditional Medicare.

At the root of whistleblower lawsuits and investigations of Medicare Advantage programs are allegations that insurers like UnitedHealthcare have gamed the system to wrongly inflate the government payments they receive.

Insurers are accused of manipulating diagnosis codes to make patients look sicker. By doing so, they generate higher “risk scores” from Medicare administrators and thus gain more federal dollars — sometimes known as “upcoding.”