More than 1 in 4 cases of possible sexual and physical abuse against nursing home patients apparently went unreported to police, says a government audit that faults Medicare for failing to enforce a federal law requiring immediate notification.

The Health and Human Services inspector general's office was issuing an "early alert" Monday on its findings from a large sampling of cases in 33 states. Investigators say Medicare needs to take corrective action right away.

"We hope that we can stop this from happening to anybody else," said Curtis Roy, an audit manager with the inspector general's office, which investigates fraud, waste and abuse in the health care system. The audit is part of a larger ongoing probe, and additional findings are expected, he said.

With some 1.4 million people living in U.S. nursing homes, quality is an ongoing concern. Despite greater awareness, egregious incidents still occur.

Using investigative data analysis techniques, auditors from the inspector general's office identified 134 cases in which hospital emergency room records indicated possible sexual or physical abuse, or neglect, of nursing home residents. The incidents spanned a two-year period from 2015-2016.

Illinois had the largest number of incidents overall, with 17. It was followed by Michigan (13), Texas (9), and California (8).

In 38 of the total cases (28 percent), investigators could find no evidence in hospital records that the incident had been reported to local law enforcement, despite a federal law requiring prompt reporting by nursing homes, as well as similar state and local requirements.

"Based on the records we had available to us, we could not determine that they had been reported to law enforcement," said Roy.

The federal statute has been on the books more than five years, but investigators found that Medicare has not enforced its requirement to report incidents to police and other agencies, or risk fines of up to $300,000.

Nursing home personnel must immediately report incidents that involve a suspected crime, within a two-hour window if there's serious bodily injury. Otherwise, authorities must be notified within 24 hours.

Medicare "has inadequate procedures to ensure that incidents of potential abuse or neglect of Medicare beneficiaries residing in (nursing homes) are identified and reported," the inspector general's report said.

In a statement, the Centers for Medicare and Medicaid Services said "nursing home resident safety is our priority and primary focus," and it is committed "to ensure these vulnerable people are properly cared for, and that all viable or alleged instances involving abuse or neglect are fully investigated and resolved."

The agency said it has long required nursing homes to immediately report abuse and neglect to state officials, and it will have a formal response to the inspector general's findings once the audit is complete.

The inspector general is urging Medicare to start systematically scouring computerized billing records for tell-tale signs of possible abuse of nursing home residents. Investigators used that approach to find the cases, matching emergency room and nursing home records.

Of the 38 unreported cases, 31 involved alleged or suspected rape or sexual abuse, or about 4 out of 5.

But even among the 96 cases that were ultimately reported to police, investigators were unable to tell if the federal requirement for "immediate" notification was followed.

In one case classified as "reported to law enforcement," an elderly woman with verbal and mobility limitations was taken to the emergency room after she was allegedly sexually assaulted by a male resident of the same nursing home. The report said two silver-dollar-sized bruises were noted on her right breast.

Nursing home staff had helped the woman bathe and change clothes after the incident. "These actions could have destroyed any evidence that may have been detected using the rape kit," said the report.

Nursing home employees did not immediately report the incident to police, although the federal reporting requirement was in effect. The nursing home "should have reported the incident to law enforcement within two hours of witnessing the incident," the report said.

Instead, the following day the nursing home contacted the woman's family, who called the police, triggering an investigation.

Citing a separate probe by state officials, the inspector general's report said the nursing home "contacted local law enforcement in an attempt to keep law enforcement from investigating the incident."

The state's own report found that the nursing home told police "we were required to report it but that we were doing our own internal investigation and did not need (police) to make a site visit...no one was interested in pressing charges." The police continued their investigation.

The state later cited the nursing home for failing to immediately notify the patient's doctor and family, as well as other violations of federal regulations. But state officials classified the incident as resulting in "minimum harm or potential for actual harm."

No other details were provided in the federal report. The inspector general's office reported all 134 cases to local police.

The number of nursing home residents is expected to grow in coming years as more people live into their 80s and 90s. Medicaid is the main payer for long-term care, while Medicare covers doctors' services and hospital care for elderly people and the disabled.