Medicare paid more than $50 million in potentially improper bills from ambulance companies for rides for older Americans, government investigators said Tuesday.

The payments included $30 million worth of ambulance rides over a six-month period despite an absence of evidence, based on Medicare records, that the patients received any care where they were taken, according to a report by the inspector general for the Health and Human Services department. The report also identified another $24 million in payments for rides to and from doctors’ offices and other destinations for which Medicare does not cover ambulance services.

Together, one in five of the nearly 16,000 ambulance services nationwide that transported Medicare patients had at least one type of questionable bill, the investigators found.

Questions of wasteful and fraudulent spending have long dogged the federal entitlement program, which insures about 50 million elderly and disabled Americans. Republicans often cite Medicare as part of their drumbeat of criticism of improper federal expenditures. And in recent years, spending on ambulance bills has appeared to be a problem spot.

Previously, government investigators found that Medicare’s spending on ambulance services doubled to $5.8 billion between 2003 and 2012. The new findings are based on an analysis of Medicare’s billing records for 7.3 million ambulance rides in 2012. The findings suggest that “inappropriate and questionable billing for ambulance transports continues to pose vulnerabilities” to Medicare’s financial integrity, the report concludes.

Under Medicare’s Part B rules, which cover outpatient care, ambulance rides are allowed when other kinds of transportation would endanger a patient’s health and when a patient is traveling to or from a hospital, nursing home or several other kinds of medical facilities.

The inspector general’s inquiry found that bad billing by ambulance companies remains concentrated in certain parts of the country, with slightly more than half of the questionable bills coming from Philadelphia, Los Angeles, New York and Houston.

And for companies with an especially large share of problematic bills, the average distance billed for ambulance transport in urban areas was 34 miles – more than three times the average trip for Medicare patients in urban areas.

In the case of rides for which Medicare had no accompanying bills for medical services, the report concludes that “the transports may not have occurred” or that patients received services that the program does not cover – and that therefore did not justify an ambulance transport paid for by the program.

The Centers for Medicare and Medicaid Services, an agency within HHS, “is pursuing a comprehensive strategy comprising of several initiatives to combat ambulance transport fraud and abuse,” said spokesman Aaron Albright. In the past two years in Houston and Philadelphia, he said, the agency has blocked new ambulance services from participating in Medicare. And under an experiment to try to control fraud, the agency now requires advance permission in Pennsylvania, New Jersey and South Carolina for repetitive ambulance rides in cases that are not emergencies.