Medicare Advantage, a private-sector alternative to traditional Medicare, was designed by Congress two decades ago to encourage health insurers to find innovative ways to provide better care at lower cost. If trends hold, by next year, more than half of Medicare recipients will be in a private plan.

But a New York Times review of dozens of fraud lawsuits, inspector general audits and investigations by watchdogs shows how major health insurers exploited the program to inflate their profits by billions of dollars.

The government pays Medicare Advantage insurers a set amount for each person who enrolls, with higher rates for sicker patients. And the insurers, among the largest and most prosperous American companies, have developed elaborate systems to make their patients appear as sick as possible, often without providing additional treatment, according to the lawsuits.

As a result, a program devised to help lower health care spending has instead become substantially more costly than the traditional government program it was meant to improve.

Eight of the 10 biggest Medicare Advantage insurers – representing more than two-thirds of the market – have submitted inflated bills, according to the federal audits. And four of the five largest players – UnitedHealth, Humana, Elevance and Kaiser – have faced federal lawsuits alleging that efforts to overdiagnose their customers crossed the line into fraud.

The fifth company, CVS Health, which owns Aetna, told investors its practices were being investigated by the Department of Justice.

The health system Kaiser Permanente called doctors in during lunch and after work and urged them to add additional illnesses to the medical records of patients they hadn’t seen in weeks. Doctors who found enough new diagnoses could earn bottles of Champagne or a bonus in their paycheck.

Anthem, a large insurer now called Elevance Health, paid more to doctors who said their patients were sicker. And executives at UnitedHealth Group, the country’s largest insurer, told their workers to mine old medical records for more illnesses – and when they couldn’t find enough, sent them back to try again.

Each of the strategies – which were described by the Justice Department in lawsuits against the companies – led to diagnoses of serious diseases that might have never existed. But the diagnoses had a lucrative side effect: They let the insurers collect more money from the federal government’s Medicare Advantage program.

In statements, most of the insurers disputed the allegations in the lawsuits and said the federal audits were flawed. They said their aim in documenting more conditions was to improve care by accurately describing their patients’ health.

The increased privatization has come as Medicare’s finances have been strained by the aging of baby boomers. But for insurers that already dominate health care for workers, the program is strikingly lucrative: A study from the Kaiser Family Foundation, a research group unaffiliated with the insurer Kaiser, found the companies typically earn twice as much gross profit from their Medicare Advantage plans as from other types of insurance.

Many of the fraud lawsuits were initially brought by former employees under a federal whistleblower law that allows them to get a percentage of any money repaid to the government if their suits prevail. But most have been joined by the Justice Department, a step the government takes only if it believes the fraud allegations have merit. Last year, the department’s civil division listed Medicare Advantage as one of its top areas of fraud recovery.

In theory, if the insurers could do better than traditional Medicare – by better managing patients’ care, or otherwise improving their health – their patients would cost less and the insurers would make more money. But some insurers engaged in strategies – such as locating their enrollment offices upstairs or offering gym memberships – to entice only the healthiest seniors, who would require less care, to join. To deter such tactics, Congress decided to pay more for sicker patients.

Kaiser, which both runs a health plan and provides medical care, is often seen as a model system. But its control over providers gave it additional leverage to demand additional diagnoses from the doctors themselves, according to the lawsuit.

“The cash monster was insatiable,” said Dr. James Taylor, a former coding expert at Kaiser who is one of 10 whistleblowers to accuse the organization of fraud. At meetings with supervisors, he was instructed to find additional conditions worth tens of millions of dollars. “It was an actual agenda item and how could we get this,” Taylor said.