The Centers for Medicare & Medicaid Services said in late September that it had canceled roughly 315,000 enrollments covering more than 760,000 people, citing unauthorized or improper sign-ups. Insurers have since identified cases in which coverage should not have been terminated, and more people may discover they have been dropped when they try to use their insurance.

The review applies to states that rely on the federal HealthCare.gov marketplace. A further 419,000 sign-ups are being examined. Insurers are expected to report to CMS around mid-to-late November how many people in that group did not respond to outreach, said David Merritt, a senior vice president at the Blue Cross Blue Shield Association. He confirmed that some of the association’s members have had coverage reinstated.

CMS selected nearly 1.2 million enrollments for closer review in June based on four warning signs: applications without a Social Security or immigration document number, broker involvement, no monthly premium payment, and no insurer contact or claims. Insurers were instructed to try at least two ways to contact affected customers and give them 30 days to provide information before their coverage was canceled.

Those indicators can point to unauthorized enrollment, including cases in which brokers sign people up without their knowledge. But they can also describe legitimate customers. A person may not have filed a claim in a year, for example, or may fail to answer calls from an unfamiliar number or open insurer mail. Some application fields may also be missing for reasons that do not make a person ineligible.

“It may be true that some of those enrollments were not authorized, but it can be equally true that people got lost in the shuffle and have a valid enrollment,” said Katie Keith, a health policy and law director at Georgetown University. She said the limited information released by the administration makes it difficult to judge how many legitimate customers were affected.

The administration has not disclosed how many cancellations were reversed, which insurers were involved, or how many people were removed for specific reasons. CMS said it had no additional data to share.

Supporters of the crackdown argue it is aimed at phantom enrollments and fraudulent agents who collect commissions. Brian Blase, president of the Paragon Health Institute, has said he believes more than 99% of the cancellations involved zero-premium plans arranged without consumers’ knowledge or enrollments for people who do not exist. Other policy experts caution that the available information is not enough to establish that conclusion.

Insurers and enrollment assistance organizations say reaching customers can be difficult. Johnny Wallin, an executive at a company that helps people enroll in marketplace plans, said his team could not reach about 60 clients on the initial lists. In one case during the next review phase, a client needed help submitting a renewed green card; language barriers and work schedules complicated the process.

The coverage review is part of a broader Republican focus on fraud in government programs. At an Oct. 14 news conference, Health and Human Services Secretary Robert F. Kennedy Jr. and CMS Administrator Mehmet Oz also highlighted the end of subsidies for about 189,000 ACA enrollees who had duplicate Medicare, Medicaid or Children’s Health Insurance Program coverage. Officials described those cases as overlapping coverage rather than fraud.

The scrutiny comes as marketplace enrollment has fallen by nearly 3 million in the first months of this year, with the expiration of enhanced federal subsidies contributing to the decline. The administration has also pointed to its anti-fraud actions and other marketplace changes. Many of the people removed from coverage may live in states including Texas, Florida, Louisiana and Mississippi, where ACA enrollment grew rapidly in recent years.

The political impact may depend on whether people who were wrongly dropped can restore coverage before needing medical care. “To the extent there is a reaction depends on whether the government got it wrong, whether people try to get care and then find out their coverage is no longer active,” said Adrianna McIntyre, a health policy and politics professor at Harvard’s public health school.