Health care became more hazardous for patients at hospitals purchased by private equity firms, a new study shows.

The sweeping study, which was published Tuesday in the journal JAMA, looked at the rates of 10 serious adverse events associated with medical care at 51 hospitals before and after they were purchased by private equity firms — a financing model designed to make money for investors. The researchers then compared those results with the rates of the same complications at 259 hospitals that were not owned by private equity firms.

Private equity firms have been acquiring large chunks of the US health care delivery system in recent years. In addition to hospitals, those acquisitions include nursing homes, behavioral health systems, and private physician practices. Academic research has shown that private equity ownership is associated with higher death rates for patients in nursing homes and increased costs to taxpayers. Earlier this month, the Senate Budget Committee announced its bipartisan investigation of the impact of private equity purchases on health care facilities.

All told, researchers analyzed the outcomes of nearly 5 million hospitalizations, which were available through Medicare claims data. Researchers had at least three years of data on each hospital included in the analysis.