If there were intentional violations of such an agreement, it would mean “that a defendant, already caught once defrauding the government, has apparently not changed its corporate culture,” said Michael Hirst, a former assistant U.S. attorney in California who oversaw the case against Tenet. Hirst now represents whistle-blowers.

In its statement, HCA said it fulfilled any obligation it had under the agreement to report “substantial overpayment.”

The revelations in the documents come at a significant time in the evolution of medical treatment in the United States — from independently owned hospitals to large, corporate chains.

HCA exemplifies the trend. In 2006, HCA was taken private by a group of private equity firms, including Bain Capital, the firm co-founded by Mitt Romney, the presumptive Republican presidential nominee. (By that time, Romney was no longer a partner in Bain.) By mid-2010, the private equity owners were eager to start cashing out of their investment. While HCA prepared for an initial public offering of its stock that took place in 2011, it borrowed to pay the private equity firms $4.3 billion in dividends.

The ability to take these financial steps hinged on HCA showing continued robust profit growth at its hospitals.

And for that the company turned, in part, to cardiac care.

Cardiology is a lucrative business for HCA, and the profits from testing and performing heart surgeries played a critical role in the company's bottom line in recent years.

Some of HCA's busiest Florida hospitals perform thousands of stent procedures each year. Medicare reimburses hospitals about $10,000 for a cardiac stent and about $3,000 for a diagnostic catheterization.

But in recent years, doctors across the country have been less quick to implant stents, instead relying on drugs to treat blockages. Medicare has also questioned the need for patients who receive cardiac stents to stay overnight at the hospital, cutting into the profitability of the procedures at many hospitals.