A U.S. government watchdog is raising fresh concerns that health insurers are exaggerating how sick Medicare patients are, receiving billions of dollars in improper payments as a result.

Health insurers selling Medicare Advantage plans to seniors and the disabled received an estimated $6.7 billion in 2017 after adding diagnoses to patients’ files that were not supported by their medical records, according to a report released on Thursday by the U.S. Health and Human Services (HHS) Inspector General’s Office.

Inspectors found that Medicare Advantage insurers had added diagnoses for diabetes, heart disease and other conditions in 99.3% of chart reviews of patient information, even though they did not appear in records from doctors, hospitals or other medical providers.

Insurers deleted incorrect diagnoses less than 1% of the time, they found.

The additional diagnoses boosted government payments to insurers by an estimated $6.9 billion, while the deleted information trimmed payouts by nearly $200 million, producing a net benefit of $6.7 billion for the companies.